EDIT: Here is what a guy from another forum found for me, seems alot of people are having this problem (Taken from http://community.callofduty.com/thread/100579227)
"There are many clans out there who compete that are running into the same problem. LoH competes in the SEALs league and we have found there are a few things that you can do to make it easier to get more than 9 in a private lobby.
1. Have everyone remove their "Elite" clan tags and title.
2. Send out the invites on the main MP screen. When you have 6 or 7 people in the lobby, switch over to a private match. This has worked for us a bunch of times.
3. I've been told that not everyone has to leave the "Elite" clan to get more people in a private lobby. Only 1 or 2 people need to leave the "Elite" clan and 1 of them need to host.
Hope these help."