Oh no, you had to take 30 seconds more in order to get in the same instance... And as far as the instance thing goes, you can join anyone, forever. Unless they're in a zone you don't have access to... In this case, your friends can give you access to the zones by going to the end of them and having you warp to them. I've logged hundreds of hours in coop, it works great...
I mean, unless you're overly bitchy/lazy, these are minor inconveniences for a free game, let alone, one that is that fun.