UserId should not be required for Groups

[sheharyarn]

user_id is one of the required arguments for the group/4. But Segment's own group docs only mention two fields; groupId and traits. This is because usually, you would use this to identify the group itself and not associate a user with a group.

This method should be updated to remove the user_id argument. If someone needs to pass it, they can use group/1.