Confirmed..needs to be invited 50 times.
My method and others:
- Make sure your throwdown challenges are turned on IMPORTANT!!!
- Go to Training (practice your counters while waiting)
- youll see a red mail in the bottom right hand corner PRESS SELECT
- Boom thats 1/50 =]
- Fight match and Quit. Rematch does not add to your 50.
- Repeat Step 1. Good luck happy hunting.