For parents or guardians to be able to add other parents or guardians to their child's Spond account, the group admin has 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, parents or guardians can edit these fields in the Spond app 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.
New parents or guardians can only be added on the Spond app, not on desktop.