In the Bosma app, go to Home Screen -> Menu (top left corner) -> Family -> Select a family -> Members -> tap the “+” button at the top right corner and enter their Bosma account email address. They will receive an invitation email, and when they accept the invitation by clicking the invitation link in the email, they’re successfully added to your Family.
Note that the users you are going to add to the family, must first download the Bosma App and register as a Bosma user, before they can accept your invitation.