Why would co-workers be in a WhatsApp group together and exclude others from it and at work?

A bunch of co workers are in a group they made on WhatsApp. This includes our Supervisor. They exclude me and some other staff from the group and in person as well at work. Me and the other staff don’t get as many hours and get the crappy jobs. Where as before, the hours and jobs were evenly distributed amongst us all.

Why would a work place do this and what would you do if you were part of the excluded staff members?
Why would co-workers be in a WhatsApp group together and exclude others from it and at work?
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