Hi there, and welcome
I can't vouch for any particular organization/agency as I've not worked directly with any but I am familiar with two of our members who utilize the services of multi-VAs within their practice. I trust both of them immensely and would vouch for their legitimacy and professionalism any day - one, Erin Blaskie, has actually recently placed an ad on the boards here
looking for VA partners, and the other is Kathie Thomas although I honestly don't know if she's taking on new VAs actively right now and she does charge a fee after successful application to join her team. You will find info about joining her company here
There are surely numerous others that I'm just not aware of, even within our own membership, perhaps someone else who does currently work on a VA team can pitch in their two cents here too
One other thing - regardless of not becoming a business entity you will likely still be responsible for your own taxes, etc. when you contract with these teams.