We have seen a number of issues recently with users being unable to invite users to their groups, and being unable to send friend requests. In a thread where we were attempting to assist exover, exover was able to contact a Game Master at Blizzard whom was able to confirm some possible server issues related to group invites.
Please keep this in mind if you experience similar issues.
You can view that thread here: https://solidice.com/forums/1/topic/oqueue-cant-invite#post-6952
Contents of post from 9/18/14:
ive sent a ticket to Blizzard, in 1st reply they told me few steps to clear addons etc etc and in second message they admit to have server issues:
Thank you for contacting the World of Warcraft Game Master Department. I apologize for the wait and that I was not able to contact you online.
If the steps we gave you in the previous response did not help, then it is very likely that your issue with the invitations is due to a server issue that we are having right now.
I looked into this and saw that others are currently experiencing the same thing. Invitations are not something we can actually fix, unfortunately. I recommend giving it a bit of time until the server issues are fixed and then try again without the add ons .
Hopefully, this will not take too long.
If this resolves your issue, please choose "Mark as Resolved" above. If you have additional questions or concerns, you may choose "I Still Have a Problem" to reply to this message.