Try modules - Role field, Rules. Create field role and assign this role from Rules module.

Try the Role field and Rules modules.

• Create a role field, field_user_role (check boxes/radio button) in admin/config/people/accounts/fields
• Create a rule in admin/config/workflow/rules, for the After saving a new user account even
• Add Entity has field > Entity: [account], Field: field_user_role as condition
• Set a data value in Actions.