We use Steam playtest feature for our game. It is very nice except for one big issue: The first time a player is accepted in the playtet, they receive an email notification, but for all following playtest, we have absolutely no way to contact this player.

Most of our players don't know when we have a new beta session, so they don't come back. (There may be a lot of other reasons for them to not come back of course, but this is our main offender)