How to use same account for multiple companies (Just like a same account can be used for multiple workspaces on Slack)

I am working on a web application which is based on Google App Engine (GAE). The requirement of the application is that one user can be associated with multiple company accounts.

My application has two main types of accounts. One is that main Admin/Company account. Other is the employee account (i.e. the persons who work under a company). Now, what I want to do is that to allow an employee to work under more than one companies, but he does not have to make a separate account for each company. His single account can be associated with multiple companies.

I have explored different platforms which are already supporting this feature. The major ones which I found are Slack and Asana. And my problem can be perfectly mapped to what Slack is doing right now i.e. I create a single account on Slack and I can join multiple workspaces on Slack using this single account.

I want to achieve the same in my application too. I am curious that how Slack is supporting this feature right now? Does it send some ID with every request to the server which indicates that the activity which just has been done is associated with the workspace under this ID? Or there is some unique sort of token associated with every workspace (on Slack API level)?

I do have such a model in my app. A unique auth_token is associated with every company account. So, I am thinking that when an employee wants to do an activity for a specific company he will send this unique token with the request to the server so that server knows for which company the activity was performed.

Does anybody know what is the best way of achieving this?