Gamezebo and BigFish have sign ups as do others. Let me see if I can find direct links.
On BigFish, you are automatically on the list to beta test if you are a club member. They send out 10K requests anmd have over a million members so you may just not have had your name pop up.
It is also done by your game history. If you buy TM games you will get asked to Beta one, if you buy HOGs only, you won't get asked to do a TM.
You can sign up at Gamezebo
here. They send out lots of requests of all kind,
Please remember if you post here on a beta you are doing and the game is not released, your post will be deleted. Betas are to be discussed only in the proper forum at the site you got it from.
Ana