How do I add people to my family plan?

Here's how to add people to your family:

Go to the Account tab in Slopes
Tap on Manage Family
Tap on Invite
Use the iOS share sheet to send the invite link

You can also remove people from your family on this screen. Doing so will immediately end their benefits of being a member of your family plan, however any activities that they recorded while they had an active pass will remain unlocked forever.