To assign Partner, Manager or Staff roles to users:
Log in to Tax.
Go to the Settings cog icon.
Click the Users tab.
Click on the required staff name.
Under the Groups tab for the user, you will see the Role column, which you can assign them as a Partner, Manager, or Staff.
Press Save.
Their name will now be in the available list when assigning a Partner, Manager Staff to a client via the client details screen.