Add or Remove Other Owners
Basecamp 3 accounts can have more than one owner, so the original owner can designate additional people as owners — handy (and recommended!) for when you're out of the office having a baby or something, and your credit card expires! You don't want to have to talk to Basecamp support while you're in labor, do you? Of course not. But seriously, adding a backup account owner really is a good idea.
Start by clicking
Adminland on the top right of the Home page just beneath your avatar.
Once in Adminland, scroll down until you see the Owner Actions.
Add/remove account owners option and add your new co-owner there.