Originally Posted by Tyger7
You participated in a contract competition
In Contract Mode, there are competitions you can play (multiple people invited). Go to the Compete menu and look at your challenges you have been invited to. If you need someone to create and invite you to these, please visit the achievement trading thread below.
I did this on my own, if you create a competition and add a couple of people etc and then do the contract/s yourself, the achievement will pop as you've participated within your own contract competition.
This is what I did as I couldn't find anyone to help me at the time.