Slack isn't all that great. As messed up as the new Yahoo Groups is you're better bet would be to start a closed Yahoo Group, where only members can see what is there.
If you were closer I'd invite you to my local Writer's Group. We've been going since 2004 and everyone that has been a member at least more than a year has a publication credit.
We got started when Dawn, looking through Critters' data base contacted a bunch of us about a face to face group. 12 years later we're still here. If you don't have local group where you meet up, you might want to try something similar in your area.
I know I prefer face to face groups over online ones. Get a lot more out of it actually talking with each other.
HM Q1 2015.
HM Q3 2016
HM Q4 2016
R Q1 2017
The Trade's On, Issue 8 of Sci Phi Journal, November 2015
Naidin's Song: Blood Bound, published August 2016