sharing online content

Can a player in a game (who is not the owner) share their online content (players handbook etc) with the other players in that game?

I believe it only works if you're the one who set the game up, but I could be wrong.  You can still join as a player on those games, though.

Compendium Sharing privileges are indeed determined by the game creator, not the GM. If your goal is to allow them to use the charactermancer, a roundabout way would be to let them create their characters in a game you create and then use the Character Vault to move them into the other game (assuming the  new game has sufficient permissions to allow the use of the Character Vault).

You (the person who owns the content) can also create the game and promote someone else to GM. There's a handy button to "rejoin as a player" for those that do it that way so you don't have access to any of the GM-only things for the campaign itself.