In order to invite friends to the game, everyone will need a free Halfbrick+ account. The quickest way to make one is to press the Team button from the game menu:

This will then prompt you to make a free account. Follow the steps to verify your email and then you're good to go! Each player will need to do this.

From the Teams screen, you can then invite or add friends to your profile, this will allow you to play together. Once your friends are in a party, hit play and jump into a match! ⚽