When it goes free, I'll be reinstalling again. As one of the three Guild Masters (...don't ask), feel free to poke me if you want an invite and you roll on the Argent realm. Characters of either faction can join a guild, although we're currently entirely Defiant.
Last I played, Argent's population had taken a bit of a nosedive, but F2P should hopefully fix that and make the game populous; it's a game that REALLY needs the population around. Unlike many other MMOs I've played, where other players who aren't in your group being around could be a pain in the arse, Rift mostly makes it better to have them there; either through Public Grouping for the dynamic events, or in regular questing, where (if memory serves) you can still get quest credit and cash for helping players that aren't in your group (you can't get drops of non-quest items)... but on the flipside, being extremely underpopulated kills the feel of the world, rather than making it feel like you've got room to breathe.