Recommendation for user synchronization between organization and external website

We are building a small social media platform for collaboration among a group of similar (but disparate) organizations. Each organization will have a small group of users who will be able to post content on this platform.

What would be the recommended ways for these users to create accounts on our social media platform? Our primary concern is if any member of the group leaves the parent firm, then his access should be revoked immediately from the platform too. He/she would obviously know the credentials, since he/she was a part of the entitled users earlier.

We are planning to host our platform on a cloud provider like MS Azure or AWS.

Thanks in advance