You can always add another player to your friend list. To do that, you need to know his/her in-game ID. In addition, you can add players to your friend list after a fight in team modes. To do this, press the appropriate button to the right of the player's nickname in the "Battle Outcome" window.
There is also another option - you can send an invitation via any social network. After clicking the link, the recipient will be redirected to the Google Play or App Store so that he/she could download the game. After that, you will be able to add this player to your friend list and start conquering the Bullet Echo rating tables together.
By tapping on the “Team” button on upper-right corner of the screen, you can create your own team and invite up to four friends (who are online) to it or join a team that already exists (you'll need the Team ID).
Your friend can share a Team ID with you or send an invitation in the game. The battle will begin only when all the players in the team confirm they are ready to fight. Teams can be created and dismissed over and over again, since there is no limit.
You can team up with two friends to play "Battle Royale" mode and up to four friends to play the "Squad vs Squad" and "Sabotage" modes.