Once you’ve reached level 18, tap the Allies button on the main Cantina screen.

Enter your friend’s Ally code in the box, then tap Invite.

You can check if your friend has received your request by tapping Request Status.

 

If you’re in a Guild, you can invite other Guild members to be your Ally by tapping on their name in the Guild member list, then tapping Ally Invite at the bottom left of the pop-up window.