I'll say this again one last time...and it's not directed at anyone in particular at all.
Don't pay attention to who signs up. Just sign up yourself and show up. Unless there are some sort of restrictions placed on the party, take the chance. It's about all I can say.
It seems to me that most people who complain about people who quest a lot or who advance quickly or whatever have a bit of jealousy invading their thoughts. Before anyone comes down on me for saying that, let me just assure everyone that I have fallen into that line of thinking in the past. I've seen people who had been here for less time blow right past me in levels, I see people who are half my main character's level who have 10 times as much gold in the bank as my character ever had at his account's peak. I see people dropping insane amounts of gold for rare items...things I can really only hope to find sometime or get as a gift.
But guess what....it doesn't really matter. The only real metric that matters is comparing your character to itself. But I tell you this, my character didn't get where he's at today by sitting back and lamenting everyone who's gone past him by some measure or the other.
What's my point? Take the chance. Sign up, and more importantly,
show up for the quests. And once there, be a part of things. It doesn't get any simpler than that.
No, you won't always get to go, but you'll never get to go if you look a the calendar, see the same names every time and think, "oh there's no room for me."
Try...maybe it won't work out this time, but at least you have a chance.
I believe I've now truly said all I have to say here. Though I've been known to be wrong about such things.