If you wish to invite people who is not a member of the group to an event, without adding them as members, you can do this. Go to the event, press "Invite" and chose "contact or email/phone number". When you add their details they will receive an event invite and can accept this.
This can be a useful feature for example at football matches when you want to register the audience.
If you invite a child who is already a member of another group, you will find the person's Spond user, but the parents are not visible to you and they will not be notified of the invitation and will not have the event added to their overview. For that to happen, the child must be added as a member of your group.