The online for this game is still very popular so you should be able to get 7 others into a game with you.
If you cant get 7 randoms in a game you can create a gaming session here
or join an already created session here
.If you are using randoms I suggest you keep starting challenges to keep them interested in staying in your game.
Once you have 7 others in a game with you there is a Freeburn challenges called 'Batter Up' which is for all 8 players meeting up at the Wildcats' Baseball Stadium, which is perfect for this trophy.