How do I generate an API key for a set of users?

I essentially want to generate an API key that allows external companies to create records for those users in our database.

Imagine our database has 9 users associated with 3 external companies. Each company owns 3 users, making up the total of 9.

However, the problem lies here:

We have…

Company A which manages 3 users Company B which manages 3 users Company C which manages 3 users

I don’t want to give Company A management access to Company B’s users or Company C’s users. Likewise with Company B & C.

How do I generate an API key for the management company, which gives them access to create records (via REST) on behalf of just a select set of users in our database?