There are a few ways to collect guest or plus one information during registration.
With Simple Registration or RSVP Registration, go to your Event Dashboard menu. Click on Setup > Registration, then scroll down and check "My attendees may bring guests." Here, you can set the number of guests that an invitee may bring event-wide.
To specify guests per invitee, go to the Event Dashboard menu, and click on Attendee List. Upload your invitation list, then, click on the attendee. Select View/Manage > Edit Attendee to update the number of guests that specific invitee can bring to the event.
With Ticketed or Multi-Event Registration, ensure that someone is able to purchase multiple tickets or register for multiple sub-events by going to the Tickets / Sub-Events tab.
Then, click on Show Advanced Options and confirm the following settings:
maximum is set to 2 or higher
minimum is set to 0
interval is set to 1