To give a user billing permissions:
Click 'Squad Management'.
Click on 'Permissions' under your Squad's name.
Type in the 'Add a squad member' search box for the user you'd like to give billing permissions to, and add them.
Once the user you want to allow billing permissions for is added, click on the dropdown by their name.
From the dropdown, select from billing 'admin', or billing 'viewer'. Click here to see the difference in permissions.