With this game's match making system, I have no idea if I'm creating a new session or joining another session somebody else has started. While this aspect in itself is fairly meaningless, it then got me wondering about all of this game's expansions and promos. Will I be able to join in online games with expansion/promo cards I don't yet own and visa versa?
The matchmaker looks at all the sets owned by all the players in the game, and creates a super-set containing everything everyone owns. It then chooses the Reserves and Environments from that super-set.
So, this means that you could play with things you don't own. This is like someone bringing an expansion to a game night. You can play the expansion until that person goes home!
You can find a few more details in the FAQ section under Ladder/Casual games.
However, as of this writing, in this phase beta there is no way to buy gold, so there is no way to buy expansions. The next phase will give some gold to active players, so you may see some of the expansions after that.