Luke,
If you are unfamiliar with UNIX/Linux: user_id and group_id are UNIX-specific identifiers as the F5 host system is Linux-based, BUT, the please do read the note on create_user method in v10.1 SDK:
Note: this method is deprecated; please use create_user_3 and associated data. (home directory, user id, and group id are now computed by the system) Note: as of v10.1.0, this method will silently ignore any of user id, group id, and home directory passed in.
i.e. you don't have to worry about user_id/group_id and home_directory anymore if you are running version 10.1+
Sam