I am sorry for spamming A_1 and Bo0 last night, but due to the delay that happened when I invited the other guys, I was not even sure you were getting them.
Ok this here answers what I suspected.If you are already in a group that invite does nothing.I was in a group with my friend Jabinski and Ancient_One last night and got no invite.
I have sent invites before and got no response,well because you were grouped up,so they need to do something a long the lines of either something goes to your inbox,or gray out the invite tab for group.
One other thing is to set the group to open invites,so anyone in the group can invite someone.There are a few non clan members that some of us have played with before that are quite fun to have along for the ride.
Then in the process maybe our ways of awesomeness will rub off on them and they will want to join
What I usually do when sending an invite is let the person know to send you a message when they are done with there current game so you can invite