Well, there's tip missions, and radio missions, and story arcs, and task forces, and so on. They're all door missions, but they're not all the same.
Safeguard/Radio missions
Regular missions from contacts
AE Missions
Signature Story Arcs
Incarnate Missions (Dark Astoria), & Trials
Faction Specific Missions Hero, Villain, Praetorian
Tip Missions
To name a few types.
Edit: Incarnate missions are VIP only, so nonsubscribers can't participate in them.
It's also entirely possible that the players joining may not know what a mission is at all, or may be used to running Death from Below trials. I find lately it's partly people in the know wanting to know what you're doing and partly people very new to the game who don't understand how missions work very well (or at all).
Iscariot's Guide to the Tri-Form Warshade, version 2.1
I'm sorry that math > your paranoid delusions, but them's the breaks -- Nethergoat
P.E.R.C. Rep for Liberty server
So I haven't play COH for a while now and I've noticed that when I'm looking for new players to join my mission team I keep getting "why type of mission team"? From my experience there is only been one type of mission...pretty much door missions. What other types of missions are there? obviously I am unaware of the other types.