If a user is a base-level collaborator, you can go to the “Share” button for the base in the top right to view, add, and remove collaborators. Note that this is different from the “Share” button for a specific table/view.
If a user is a workspace-level collaborator, you go to the Workspace settings screen to add/remove collaborators. Note that removing a collaborator from the workspace will remove the user from all bases. If the user needs to retain access to other bases in the workspace, you can then add them to those individual bases.