I have done something similar in the past earlier versions: will need new modal form/field to show users front end, extend registration table/model, controller and event view by adding ability of allowed users to select another user for attending. Might need additional registration related fields and validation.
Will also need change to back end registration to mirror additional fields and visibility who registered whom.
Will have to decide now who can delete/amend registration: if i register you - can I de-register you or not? Can I de-register anyone from front end?
You only have to give permission to the allowed users to enter in the attending list from event view.
I have this and it's very easy to add (only 2 lines in view.html.php)
in addition it would be good to ad an fa-external-link image to the number of booked places, if a person has the rights to add users. (Most peoples don't see the link on the number, you can click)
But you can only link to the attending manager, when you are attending.
Testing JEM 2.3.0 beta3 local on clean latest Joomla (3.9.11) Protostar php7.3.8 (no mails)