If you wish to invite people who are not members of your group to an event, without adding them as members, you can do this.
Go to the event, press 'Invite' and select '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, when you want to invite supporters to attend.
NOTE: If you invite a young person who is already a member of another group, you will find the person's Spond user profile, but their parents/guardians are not visible to you, 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.