connect users to Custom post types

Hi,
I’ve created a Custom Post Type to manage Events and now, i would like the users of my site (only registered ones) to be able to register to any events created. On the events page, they would just click a button and they would be associated with the event (no need for tickets, payment,…).
I have no clue on where to look at to implement that (taxonomies, meta, … ?) and how this will be stored (on user entity, on event custom post type entity?)
I need help !

Thanks