When an external centre invites you to nominate for their event, any super users will get an email stating this and asking you to review and accept the invite from that centre.
The steps to approving look like this:
1. Log into your centre and click on the Event - Calendar
2. Then click on the Nomination Invites option
3. Then click on the Action - Dropdown - View/Edit & Accept
4. Then set your Access to - Centre and Family then Approve Details and Submit
5. This will let your families nominate into the events offered.