Hi,

it seem to bo a compatibility issue between register_user and user_groups:
if a group is selected by default in admin / options / plugin / register_user (ie: Default user = newuser group), this group is not automatically assigned to a new user visible in admin / users.
In this case, User group membership = No group selected

what do you think about it ?

Are you sure that you're not applying a template? Users can be a member of a group, but not a template. (The default newuser is a template, not a group.)