I'd like to be able to pay the users of my site using PayPal Mass Payment. I think this is pretty straightforward if they have a PayPal account. 
However, if they do not have a PayPal account, is there any way to have them sign up through my site, without leaving? Or just with a nice redirect? Whatever is least friction. I just don't want to lose users in the onboarding experience. 
This is analogous to Stripe managed accounts, but I'm not sure if PayPal has such an analogue.
PayPal doesn't have a way for people to sign up entirely through your site, although there are some ways to facilitate the process. You'd probably have to call PayPal to get access to some of those as they are aimed primarily at larger businesses.
However, don't neglect the easy/automatic assistance that PayPal gives you: you can pay to any email account, and if that email is not already active on a PayPal account then the payment will be waiting for them when they activate that email into an existing or new PayPal account. So you can onboard merchants to your site and leave the PayPal signup for later, when money will be waiting for them to claim. Psychologically, walking away from money is harder than than deciding not to start processing :).
managed paypal accounts with least friction?