New users automatically have access to the company they have been loaded against. To give access to more companies, or to restrict access at a later time, open the company by selecting Settings from the green main menu, then Manage Companies.
Select the relevant company.
Scroll down to Security and search to add Authorised Users who can access that company with their login.
Alternatively, to remove a user select the 'x' next to their name.
Select Save to secure any changes made.