Multiple roles based on usernames

Hi there, 

I have an odd situation. I have a couple of roles setup namely Management, Operations and Brokerages.

The Brokerage role has a look at the USERNAME() sent through from the embedded application for the right brokerage and then filters according to that. 

One of the managers recently requested that a user should be able to be a member of multiple brokerages.

Is there a way to filter the DAX expression by splitting the string of USERNAME() and running a OR clause? Or how should I go about this.

example:

So let's say John is part of Brokerage 1 but needs to be added to Brokerage 2 as well but not see Brokerage 3's filtered dashboard.

Normally I would send through 'Brokerage 1' ass the Username and the dashboard would be filtered to only show Brokerage 1's results. This is achieved by using the DAX expression [Brokerage] = USERNAME()