Thanks for contacting World Warfare Support.
If you are in the same league, simply click on the league badge icon 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". (Note that there is a chance you can join if the battlefield isn't full.)
It is better to join a "Quick Match" at the same time if you wish to be in the same battlefield with each other.
League members can form their own league team to get matched into a battlefield together!