Yeah they disabled it because farmers/ad-spammers were making lv1 alts, /who'ing a zone and inviting all of them one by one and spamming their ad in partychat as soon as you were in; I got hundreds of the invites and declined every one of them(why the fuck would I join a random group, let alone one sent by a lv1 in Teldrassil who I don't know, /who out of habit ;) ) so they just removed the ability to -send- group invites on Trial accounts; paid accts can still invite Trials to groups, though.