Hello Bmxer133,
I am sorry to hear you are having this problem. I personally recommend that the Clan leader first go into Manage Clan and clear out any pending invites. Once the inviting Clan invites are cleared out, the easiest way to send an invite is to go directly to a members MW3 Playercard, and select "Invite to Clan." If these steps do not work, then this could be the result of those more than cached files.
However, please be aware that @CallofDutyElite tweeted earlier:
"Accepting clan invites is a known issue for some users and we're fixing in the next update."
https://twitter.com/#!/CallOfDutyElite/status/195025651814764544
