When entering Multiplayer (Xbox360), there are times when there are 15 other players online in my session. Some formed up in Posses, some not. If I just go off and try to complete a Gang Hideout (or several) on my own, it seems that the number of people in my online session continues to dwindle until ultimately I am the only one left in the 'lobby'. Is there a way to ensure that the 'lobby' repopulates with other players without jumping out and then back in?
This is a hard question to answer because there's no real solution to your problem. My suggestion would be to try joining one of these posses or inviting friends to join the game.
My reason for this is that Xbox Live is a sociable gaming platform where people play together so going off on your own is potentially discouraging to other players who are more focused on collaborative gaming.
You're not actually doing anything wrong it's just that certain players want different things from their online gaming experience.
Hope this helps.