Soumaya,

You can override the create method of member object and with the creation of each member object, create a new user..

NOTE: tag each user with the member for future references.. (like relation between res_partner and res_users, where res_users is having a many2one to res_partner, same way u can add a many2one field referencing your member object in res_users).

and remember to tag the member with the new user created..as

class res_users:

    member_id = fields.Many2one([member_object])

class [member_class]

def create(self, vals):

            res = super([member_class], self).create(vals)
           # Create your user here and tag res.id(new member created) to it at member_id field.

           return res

Hope it helps!