All the answers are for ver.1.7

I try to find the paths,files but all is changed at version 1.8 !

Please give me a link of this answer for the version 1.8

Your question was answered jsut a while at http://community.elgg.org/pg/forum/topic/766150/avoid-create-a-group/

You will need to do some code work... No one seems to have coded this for v.1.8 yet - maybe you can be the first and then do post your code back here ;-)

For the sidebar :
---------------
/mod/groups/start.php
line 161

if ($user) {
                if(isadminloggedin()){
                $url =  "groups/owner/$user->username";
                $item = new ElggMenuItem('groups:owned', elgg_echo('groups:owned'), $url);
                elgg_register_menu_item('page', $item);
                }


For the button :
--------------
/mod/groups/lib/groups.php
lines 15,104,135

if(isadminloggedin()){
    elgg_register_title_button();
    }


and
line 160

function groups_handle_edit_page($page, $guid = 0) {
    admin_gatekeeper();

@Z - you'll need to do some **php coding to achieve that ! ;-)

@DhrupDeScoop - I found this http://docs.elgg.org/wiki/Gatekeeper but i dont know php :(

Can you help me, provide me this code ? 

u r guessing ?
not gatekeeper

try :-
    if (isadminloggedin()) ...
if you "don't know php' ;-( 
it's gonna be quite very hard to *code what you want..
but you can always learn ..