Playing with friends is always loads of fun! You can easily create a party for your friends to join or join theirs instead. Here's how to do it:
Tap either buttons in the battle menu to open the party window.
Invite your friends into your party by tapping "+Party". If your friend invites you to their party, you can see the invite in the "Requests" tab.
You can also share your party's code (at the top of the window) with any player either by copying it or sharing as a message, or add a code from another player's party in the text box and tap "Join".