The Guinea Pigs are not a full faction, so they can also become members of another faction. However, if you sign up to be a Guinea Pig, you are agreeing to help the admins test new features even when if may interrupt your current task, as well as agreeing to be generally active on the server. Most Guinea Pig events are planned ahead of time though so it shouldn't generally intrude on your tasks.
If you miss too many events as Guinea Pig you will be removed from the Guinea Pig faction and you may not be allowed to rejoin for a period of time.
Guinea Pig specific updates and news can be found on the Discord server.