Feature Request: Allow Users access to more than one company

As an admin I can navigate between different companies and invoice from each one.

However, my sub-users can only be invited to a single company. There should be the option to invite existing users to a different company.

When I manually invite them it tells me the email already exists. I was told the work around is to use email aliases, this is not a great solution because my staff would need to login and out of an account to see the different company.