I have an issue with my internet connection, and get5x logged off sometimes. When I come back on, everyone is at the entrance of the instance except one. to keep going, I'll invite everyone to that characters party, and then exit/ reenter with the ones at the entrance and continue on.
It's annoying, but qwest (SUCKS) seems to think since its intermittent, i'm either lying about it, or its not really a problem.