categories

As I've explained users do not join nor are they assigned to categories. GJ categories are just containers for GJ groups. There is no assignment to be made. I guess you can use Joomla usergroups then restrict each categories group create access to said usergroup so only those in that usergroup can create groups in that category. I'm not quire sure you understand the purpose of GJ, please see the below link.

www.joomlapolis.com/cb-solutions/groupjive

Hi krileon, how are you today?
I read the description of the link that you have suggested and then I have developed a new solution:
would make sense if to divide users in 10/12 categories I use the member lists?
Creating 12 specific categories in joomla, I could then assign it setting the configuration of each member list.
I already added to the registration form a dropdown menu (consisting of about 12 entries) where the user will select his own specific category. Could it be configured as a dynamic field to automatically insert the registered user in the category chosen by himself?
(It would be better to be inserted directly into the list but I think that is not possible)

Thank you for help

If you want to assign users to Joomla usergroups based off a CB field then please see the below tutorial detailing such usage.

www.joomlapolis.com/support/tutorials/107-use-cases/18237-assigning-usergroup-based-on-cb-field-value