Just to add to what Nagoya said, don't forget to add a disclaimer, "hey guys, I may have to leave at any moment because I'm on daddy duty for a sleeping baby / I'm a paramedic on call / I'm a wanted criminal hiding out from the police" when you join and people will be even more cool if you have to leave at the drop of a hat.
It's actually pretty normal for people on the 'xp highway'* to join/drop groups at a high turn over so you needn't feel bad as reps are never too far between (and even bad group compositions can thrive).
*oasis, mistmoore, city of mist, karnors castle.
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