In addition to the comments from Nic. If you are using an internal User Management Directory (eg LDAP/AD) you may wish to isolate their usage to only Confluence and so the users and the group could be created in an Confluence internal directory rather than your company-wide User Directory.
Add the new users to the user list in Admin -> user management.
I'd recommend putting the users into groups that tell you what company they are is as well, for tracking at least, although it would be useful for space admins to be able to say "let company X see my space" by adding the groups too.
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