A group of workers joined to protect and promote their interest?

This is commonly referred to as a worker's union.

The group usually elects a group of representatives to deal directly with the company/management structure of the place they all work, or trade they are involved in.

The group of workers usually pay fees or dues to the union organization for their representation/efforts on behalf of the workers, operating costs, legal fees, etc..

A union has both good and bad effects on a company and it's workers. I recommend looking into the pro's and con's of a worker's union before deciding to allow/vote one in where you work. Both the company and you will reap in the benefits they can bring and also suffer in the consequences that may come about in the future.