This article explains how you can add an existing user to your organisation.
The Member Admin of the company can add existing users who also have rights at other companies to the organisation. Select the "Users" menu and click on "Add existing user" in the top right-hand corner. Enter the name of the user you want to assign in the search field, press Enter and click on the name. Select your company in the Primary organisational unit and click on "Assign" on the right. Give the user the necessary roles and finally click on "Assign user* at the top.The user you want to add to your organisation will receive an email with a login link to accept the request.