Hi! Hmmm, that’s a toughie. I’m familiar with Facebook groups, which are by far one of the easiest ways to do a writing workshop group. Tumblr is a little trickier. I would say to advertise it, then message people you think might be interested. Then create a special tag that’s just for the group. Then, when people have something they want critiqued, they just post it with that tag. So whenever anyone is ready to critique and stuff, all they have to do is tag the author and then use the special group tag.
I would recommend keeping the group small, because it can be overwhelming to keep track of 20+ people who need critiques. Definitely no more than 10 people, if you can help it!