If u sign up to ubi does that mean you will automatically be considered for game betas? What factors affect the chance of being picked to try a game beta?

It will vary dramatically from game to game. For many of Ubisoft's games, they are only the distributor / publisher, not the developer. It is the development house who are primarily responsible for the beta test program.

I'd suggest you keep an eye on the official web page and forums for the specific games you're interested in to see if public beta programs are announced.

For closed betas you would need to have a lot of experience in beta programs and be known to the developers to be considered. A background in game development would help.

In any event, beta testing is not nearly as much fun as it might seem. You will find that the game is still in beta because it still has bugs which interrupt play frequently. You are expected to record exact details of what was going on when the problem occurred, your system details and whether or not you can re-produce the issue every time you encounter a problem (unless it's been declared a known issue). You will also be expected to be able to trouble shoot to the point of eliminating your own system as the cause of the fault, or identify system settings that may contribute.
In short, beta testing is more work than fun.