By default, if Company Access groups aren't created, all users will be able to access all companies.
However if you have users with limited company access (ie; your clients) in Backup Radar, this is controlled by Access Groups (for more on this check out Company Access Groups).
If you want to grant one of these users all companies access, you'll need to edit the user via the Security section.
Go to Security > Users and then click the pencil icon to edit.
From here you should select Remove All next to the Access Group drop down. By removing all access groups, you're giving the user access to all companies.