To allow those on your invitation list to also bring guests, please use Simple Registration or RSVP Registration.

Then, go to your Event Dashboard, then Setup > Registration, and scroll down to check "My attendees may bring guests." You can limit number of guests they can bring.

With RSVP Registration, you can specify plus ones per invitee. Go to your Attendee List and click on the attendee. Then, you can edit the amount of guests that Attendee can bring.