Add a user as custom field of CPT

Intro info:

I have a Custom Post Type (CPT) called Service. I am already connecting users to this CPT using the plug-in ‘Posts 2 Posts’, this is a many to many relation, and these users connected to the CPT represent the ‘team’ of said Service.

Alright so here’s the headscratcher: Now I want to create a custom field in the CPT Service, which will be the Director of the Service. Say a field called ‘Director’ in which the value is a User. Ideally, the value of the field Director is a User, not a string representing the user name or anything like that.

I have no idea how to start tackling this issue and some brightness on the subject would be strongly appreciated. Thank you!