Pass authenticated user from another site to sign into WordPress site.

Hello,

I have two websites:
– site.com (Non WordPress site)
– subdomain.site.com (WordPress site)

We are planning to have users login through our site.com then click a link to come to our subdomain.site.com without having to log in again all under a single WP user ID.

I’m just the project manager on this so I would need to pass on the details to our developers.

So can anyone:
1. Let me know if this is possible.
2. What calls/API I would need.
3. WordPress keys / User details for this to work.

Really appreciate any guidance here.