Thanks for your inquiry,
  • If you are in the same league, just click on the league badge icon located at the bottom of the main screen and select "Battlefield". Find the one you want to join according to the battlefield ID, then click "Enter Battlefield". There is a chance to join if it isn't full. 
  • It is better to join a quick match during the same time if you want to be in the same battlefield with each other.
  • League members can form a league team to match into a battlefield together.