at the moment I have 2 users: 1 administrator and 1 standard

I would like to know if it is possible to add a 3rd user (paying of course) but I would like to create this 3rd user in a way that he can add/view/modify his creations (as a standard user) but with the addition that I (administrator) want to be able to give him access to certain accounts (only if I want to share the contact with him)

I have been trying to understand how the permissions work but all I can find is "either he can see all my accounts o none at all"

Basiclly I want to create events attached to an account and have this 3rd user follow up on it... but he of course needs to be able to see the accounts info and eventually modify the event.

is there a way to create a rule that will permit him to access my chosen accounts?

Thank you very much in advance