Allow registrations to be for multiple people -- e.g., adding guests or "plus one"

Problem/Motivation

Allow a person registering for an event to register for more than one person -- in other words, allow the person registering to add guests or a "plus one."

For example, user registers for Event A and there is a field available during registration that asks "Will you be bringing any guests?" The user answers "2" and so after registration is complete, there are 3 spots taken toward the registration limit (the user plus their 2 guests).

Another consideration could be allowing admins to limit how many guests any given registration can contain.

Proposed resolution

Add a boolean field to the event-creation process that toggles on "Add Guests" feature for registration.

If "Add Guests" feature is turned on, add a "Maximum Number of Guests" number field to the event-creation process that allows admin to set maximum number of guests per registration.

If "Add Guests" feature is turned on, users will see an option to add guests when filling out the registration form and the options will be limted by the "Maximum Number of Guests" field set during event creation.

When a registration with guests occurs, the number of available registrations for an event should reduce by one plus the number of guests chosen.

When a registration is cancelled, the number of guests should be added back to the registration limit

Something that could be awkward... what if there are 2 spots remaining for an event that has the waitlist feature enabled and someone tries to register themself plus 2 guests. That would be 3 registrations total and so there would not be enough spots available. Should the single guest be put on the waitlist, or should the user just be told that 3 registrations will not fit and only 2 can be registered?