I'm unusual in playing at pretty much all times during any given week, but even so, there aren't so many that any half competent guild leader wouldn't know who is playing everyday, even in the hours they aren't.
As far as being able to get into a guild. It's true anyone can get into a guild. Mostly because anyone can form a guild. Getting into a guild that actually has a reliable source of people to play with is another story. How many of those invites come from established guilds with leaders that know how the system actually works? How many of those guilds are still around the next time you pop into the server?