User management happens from Xero Practice Manager. You can add, edit and remove users in Xero Practice Manager and this will synchronise through to FYI.
To add, edit and remove users in Xero Practice Manager, go to the Business tab and select Settings, then click on Staff. Select Invite Staff, fill out the required details and click Save.
Any changes will synchronise through to FYI twice a day, but if you needed, an FYI administrator can manually synchronise. Refer to Xero Practice Manager (Synchronising Clients and Staff).
If you need to restrict the usage of FYI to a subset of your Xero Practice Manager users, this can be done by making them inactive in FYI. Refer to Managing Users.