Are you sure about that? Before i get into my SG, half of my heroes get SG invites at every log-in. Both from team mates or blind.
Nearly every night i hear someone calling people to their SGs. I guess people will notice you after they shake off the impact of the I9. Also in here we have some SG topics. Just write to them, they will answer you.
In the end you can't blame people for not inviting you. There is no rule about it. Just be paitent. 
If you would like to join my SG and are on union then just send me a tell @Nicky, Im online most nites and we are a very friendly social SG with TP pads to everywhere!
well after 30mins of /b looking for SG i give up with no1 answering and start /tell ........ "are you in a SG and if so may i join" yet theres loads of people playing
this dont just happen to me on heroes but villians aswell
... is it just me do do people not want a scrapper that can not only TP himself but team member's has full flight and group flight in there SG ??