You can only add members that have already created their own free Peak account. Users who have purchased a Pro subscription on their accounts cannot be added to the Family plan.
To add a member, just open the app, go to the Account & Settings menu/gear icon in the sidebar, then tap "Family Plan" and finally "Add new member." You will need to enter the email address associated to their Peak account.
Note also that the login system is case sensitive: if the person you wish to add typed their email address with lower and/or uppercase letters, you will need to type it in the same way in the Family Plan menu.
If this did not work or there is no email address attached to their Peak account, it is very likely that the person you try to add created their account via the Facebook Login Option. This will only add their Facebook ID to their Peak account but not their email address, this is unfortunately why you are not able to add them via their email address. In this case please send us an email and we are happy to help!