it's difficult to select more friends while inviting to a group... here is an easy trick
step 1: when for example u hav made a group and want 2 invite every1, u click on" invite people to join"
step 2: when the page comes up with ur frndz list , in ur browser adrress bar( ie, the box in which u type www.google.com) , in that box copy and paste xactly the following line without adding anythin on ur own.. just copy and paste
step 3:after u copy and paste the line i gav above , hit the enter button.
step 4: wait 4 5seconds for every1 to b automatically selected