I asked that same question to the helpdesk and they replied saying that CS Pros is an Upwork Group and they told me that you cand find it and join the group on the resources page, the problem is that I cannot find such page... not very helpful really.
The Pro Customer Service program is an invite-only program for experienced Customer Service professionals. Group members were invited based on their work experience in Customer Service or Technical Support and have passed a vetting process that includes a profile evaluation, assessments, and behavioral interview.
Currently, we're sending out invitations in batches and are looking to gradually expand the number of invited freelancers to the program. This means that qualified freelancers may receive an invitation at different time periods.
You can get more details about the Pro Customer Service group on our help page here.
I would actually like to joint this group. I was using Upworks when it was Odesk over 6 years ago. I found my job on here. I worked there for 6 1/2 years. All I did was CS and I was good at it. Unfortunately, due to a take over at the business, I am back to looking for work again.