Not sure if it's on the wiki yet, the solution to not seeing your friends online when they are, in fact, online. A bit sloppy, but it works.
"I know this happens after playing a game (whether normal or practice), when one of the person you played with is on your buddy list. It seems that, after showing the scoreboards and end-game chat room, clicking on the 'return to lobby' button makes the game consider you've left/logged out. I don't know how pvp works in the background, but it seems that it's not rejoining the right channel when it should be.
If you want to quickly fix this, try to start a normal or practice game. The lobby MUST be started in order for you to appear as back online. This means you have to go further than the first 'team size, map choice ...'-screen. Still annoying but not as tedious as restarting the whole thing..."
> Create Custom Game
> Go to Lobby (no need to go to champion selection, just to the section where you invite people or add bots)
> Back out
> List back to normal