You can be provisioned with the User Administrator role if you are an internal user (your user account is established in your native tenant). If you're a visiting user (you are assigned to a workspace outside of your native tenant), the User Administrator role isn't available to you.
You can manage user access to workspaces in three ways:
A user administrator can create users, add them to workspaces, or delete them from workspaces in Administration.
A workspace administrator can add or remove users from the Users pane in a model.
A workspace administrator can import a list of users to add users and update user details in a model.
If a user administrator and workspace administrator input user changes that conflict, the most recent transaction determines the user account status.
To avoid user status conflicts, we recommend that your organization use the User Administrator role to add or remove users. Workspace administrators can then refine model-level access from the Users pane in a model.
Only user administrators can add or remove users from the Internal page in Administration. They can also invite or remove visiting users from the Visiting page in Administration.
Workspace administrators can't add or remove users from within models. They also can't add users through an import. However, they can run an import to update user attributes.