Speaking not just as a Dark/Dark Defender, I would say always follow the Tank.
However once you see the team begin to split, then let the others know as soon as possible. As a Defender you will see this well before any other AT, except maybe a Controller.
You'll probably get conflicting advice on this but a split team is a recipe for disaster. Once the call from you goes out everyone should disengage and reform on the Tank.
Mind you.... If your team has no Tank and splits..... Run 
Proud member of FOXBASE ALPHA and coalition associates.
Hero 50's - 25
Villain 50's - 1
The tanker is the one thats supposed to be supporting you!
Yes you get support too!
Using taunt control the tanker should keep all the foes in the debuff zones you create thats meant to add survivability to the team and prevent you from getting aggro.
The controller could lock down your anchor and all the people in the team could say "hey no need to attack that one he is not attacking us yet!" which is easier than "which one dont we hit, i cant see!" or to have Mr MA Scrapper 2006 come in and sidekick him away from everyone.
But alas this is vastly too complicated for the vast population of the city of mercs.
I love dark defenders good luck with it, welcome to a frustrating life!
He will honor his words; he will definitely carry out his actions. What he promises he will fulfill. He does not care about his bodily self, putting his life and death aside to come forward for another's troubled besiegement. He does not boast about his ability, or shamelessly extol his own virtues. - Sima Qian.
I have found it better to lead and let others follow my D3.
--
@Tenebra
Yes upto certain amount of +levels dark is very good at tank maging and some say they are better at tanking with their dark defender than their tanks. Which isnt strictly true as tanks redirect line of fire and keep it but the job gets done. The dark defender is part controller in a very good way. But you still have to get into some peoples heads to leave the anchor alone and against some enemy you may need the support from other people for things like clearmind.
Stormies are another good set to have as point especially when bubbled or forted. Basically many different combinations work well in teams. No AT is a definate requirement by design. I do tank mage +4 with mine but not without help.
He will honor his words; he will definitely carry out his actions. What he promises he will fulfill. He does not care about his bodily self, putting his life and death aside to come forward for another's troubled besiegement. He does not boast about his ability, or shamelessly extol his own virtues. - Sima Qian.
Thank you all of you.
That helps. So far, I've found that people ignore me when I say keep together, in range for me and I'm going for a personal record in losing anchors.
3 seconds at the moment!
Witch Hazel- level 41 electricity/electricity/electricity blaster
Doctor T. Tree- level 50 dark/dark/dark defender
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Thank you all of you.
That helps. So far, I've found that people ignore me when I say keep together, in range for me and I'm going for a personal record in losing anchors.
3 seconds at the moment!
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3 seconds, eh?.... Just wait until people start using Nova's 
Proud member of FOXBASE ALPHA and coalition associates.
Hero 50's - 25
Villain 50's - 1
When you get to 50 and want to make a new alt, don't pick a Rad Defender or Controller you'll be having anchor problems all over again
PS- I don't know about the Dark powers but as a defender, yes, you will be able to see when teams split quite quickly. In most cases, just say team's split, then go to the tanker. People learn from their mistakes. If they run away from the tank and defender they'll learn to keep close after the first or second time they snuff it...
on the plus side, I've only had one set of "heal plz", "PlZ", "HEAL".
From reading these forums and listening to defenders on my team complaining about bad experiences, I thought that would happen constantly. I haven't been asked, "cn u heal?" either.
Had someone who apparently assumed I was an empath and didn't realise I had to be in range of enemies to heal.
Pretty good so far!
EDIT: I'm security clearance 22
Witch Hazel- level 41 electricity/electricity/electricity blaster
Doctor T. Tree- level 50 dark/dark/dark defender
Indeed, on some teams I consider it a job well done if the anchor isn't dead by the time DN's animation has finished.
Who to follow really depends on the situation.
If there is multi-group agro in the same room, I try to DN one lot, and hit the other with fearsome stare. Once that emergency procedure has been done I work out what Fluffy is focusing on and go for the other group, possibly TPing him to where I want him if he's faffing. Fearsome Stare is particularly good when a second group accidently aggros and no one is shooting them, as in those cases it works pretty much as a long duration hold that can be reaplied until the team has dealt with the first group.
If, however the team have gone totally whacko and split completely (and not in a planned, "we can cause more damage that way" sort of way) you have a number of options.
1) Ask them not to. If they don't quit and go for a less suicidal team.
2) (against mezzing enemies) Follow the tank. They take all the mezzy nastiness, and you make sure that they can't die. Together you are pretty much immortal. Finish off the group you are attacking and then the defender/tanker combination of godliness can go and find the bodies of the others and you can use Howling Twilight and get some nice progress on the next healing badge.
3) (against non-mezzing, non-Nemesis enemies once you have got Fluffy) Pick a third group away from the others and solo. Once you get past a certain part of the game, as a D3 if an enemy can't mez you, you can take on huge amounts of them at once if handled properly.
4) (against psi enemies) You are the one with the group psi resistance power. Be bossy.
Dead Calm's Defender Manifesto
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From reading these forums and listening to defenders on my team complaining about bad experiences, I thought that would happen constantly.
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lol! CoH/V Veterans like to over exagerate sometimes...
What should I do with non-mezzing enemies before level 32? Follow the tank, again? Sorry if it's a stupid question but I'm much more used to being a blaster.
I would love to be able to leave bad teams, but I feel mean and unheroic. 
Witch Hazel- level 41 electricity/electricity/electricity blaster
Doctor T. Tree- level 50 dark/dark/dark defender
It's generally much safer for the team to follow the tank. If half your team keeps splitting and going off on their own then don't worry overly much in trying to keep them alive. They will soon learn not to keep wandering off.
That said, team splits can happen for other reasons. Just a matter of weighing up the situation. If the tank looks happy, go help the others. As Dead Calm said, Feasome Stare is a great power for locking down a group for a while to give you chance to go somewhere else. Darkest Night is also great as you can leave it running and be miles away and still helping. I would do that with the tanks group however as at least the tank can keep the aggro off you that DN causes.
I'm a dark/dark defender and quite often my team splits up, meaning that they are out of range of Twilight Grasp. Also, I prefer not to have to divide my debuffs between groups.
When I pick who to focus on, who should I choose, in general?
In specific incidents, it's been easy to tell (tanker who took no defences, and so needed my debuffs and heals most), but which AT(s) should I normally assume needs me most until I see evidence to the contrary?
Sorry if I've been unclear.
Witch Hazel- level 41 electricity/electricity/electricity blaster
Doctor T. Tree- level 50 dark/dark/dark defender