Adding someone to your account in the two-person collaborative mode means you can work together towards common goals. If you create a shared goal and aren’t saving from a joint account, you’ll be prompted to invite your app partner to save for the goal with you.
Once you create a goal, you can invite them to start saving with you in the goal details section. To get there, tap on the goal and hit the pencil icon in the top right. You should see a button to invite your collaborator at the bottom of that screen.