Thank's a lot, Rick, for your response.
On the Admin, login page, i have forgotten to select "advanced".
Now, it's ok: I see the option "users must choose a group at registration" and i have selected this option.
But, when a new user want to register, the group is not asked !
At registrating, I have 3 errors (see screen copy). how can i solve these 3 errors ?
Thank's for your help.