As a minor update:
CoH Channels for Cryptic Games --
It's [CoX STO], not [Cox STO]. Not trying to be picky, but channels on Cryptic games are case-sensitive.
CoH Guilds in other games --
Champions Online --
Paragon Dawn is the merger of other CoH Champions Supergroups: Justice Girls, Hero Dawn, and Geek's United. They even have a website and a radio station. Other SGs from CoH players are out there, but they're mostly quiet. Paragon Dawn also has a fleet in Star Trek Online and a Guild in Neverwinter if you're interested as well.
Star Trek Online -- The original fleet (
Paragon Fleet) is defunct, so two more were formed with active players:
Paragon Expeditionary Fleet for active players, and
Paragon Vanguard Fleet for casual and "secondary toons".
Neverwinter -- I'm not aware of a particular City of Heroes only Guild. The group I'm with is the
Enigmatic Order of The Cape, from Cape Radio, and most folks from Cape Radio are ex-COH players. So it stands to reason if you want to play with other COH veterans, that's a good option. If there's a specific COH-only guild that's active in Neverwinter, I'm willing to learn of it.
All three games, the best path to an invite is to ask in the [CoX] channel. If no one from the guild is on at the moment, try asking later but don't give up. Keep trying, someone who's in a guild will eventually hear you.