Pardon me as I rarely use/used the register/user group stuff so I am less familiar as well. I was sure there is such an option as well but even I cannot find an option to assign a default group within register_user. All that is there are options to set default rights but those don't let you assign a specific album etc.
user_groups has no options at all but a group called "default". Even to me the only way to assign anyone to a groupd is either via the assignments or the user account itself.