Hi Hanane,
You can modify on this following file email content :
JoomlaFolder\components\com_easysocial\themes\wireframe\emails\html\registration\auto.php (after registration auto login)
JoomlaFolder\components\com_easysocial\themes\wireframe\emails\html\registration\login.php (After registration manually login)
JoomlaFolder\components\com_easysocial\themes\wireframe\emails\html\group\created.php (after created group, notify to admin)