http://www.tek-9.org/forum/call_of_duty_series-46/call_of_duty_4-13/the_mix_me_thread-97113.html?page=1

From the official stickied Tek-9 Networks post.

This group is obviously meant for people who are avaible and willing to mix / merc for other teams.

Everyone can join, however, you need to be avaible now and than instead of rejecting teams to use you as a merc.