Setting up User ID for a group of people


I have a question about User ID.

Users of my platform can be grouped in clusters, thanks to an id they have in common.
User 1, 2 and 3 share a mutual id and belong to cluster A, users 4 and 5 share another mutual id and belong to cluster B, etc. There are hundreds of clusters. Each cluster is unique and has its own id.

Is it possible to set up the User ID for groups of people ? Or is it mandatory that the User ID is for a unique visitor ?

Thank you.