For guardians to be able to add other guardians to their child the administrator of the group have to enable this by going to the group settings and to 'Member fields'. Here admin can go into each of the fields to select who has permission to edit them; Only admin or Admin, Guardian and Adult members or Admin, Guardian, Adult member and Children.
Once this is enabled in the group settings, guardians can edit these fields in the app (not web) under their profile tab (top left corner) > Group Memberships > select group and member and press the plus sign next to their name to add the details. Whoever is added will then receive a notification of this. If this is not available, a group administrator with permission to edit group settings, has to open group settings -> member fields -> guardians -> guardians can add other guardians for their own children.