Registers a new attendee for the event. Validates submitted fields against the chosen ticket type's custom field definitions, processes workshop enrollment, purchase items, coupon codes, and payment initialization.

Group registration: When the attendee's ticket type is configured with allowed invitee types, registering that attendee automatically creates a new group with them as leader. The response meta object then contains group_uuid and a profiles map describing which invitee types have seats available. Additional attendees can join this group by passing group_registration_uuid (the group_uuid value) in their own registration request -- their attendee_type_id must match one of the allowed invitee types with available seats.

Supports both user and attendee (sponsor-delegated) authentication.