For current build:
Just sign in to the game with your name with the Plus memberhsip, tap on your name on the menu menu screen and choose "add user"
For old builds:
Select Your “MainUserName”
Select Add Names.
You can create a new user name and select ADD. or you can click where it appears red in the image below and add an already existing user name.
You can “unequip” a user name as well to make room for a new one. This can be undone at any time by following all of the instructions presented here.
Click the existing user name.