I'd think outside just people on atom/slack. Think about who they are as a person as a whole. ie. What do these people do when they are bored, what type of content do these people read, what platforms do they frequent, etc...
Once you understand your user as a person (keep in mind they are probably pretty similar to you) then it will be very easy to know where to find them. Just because it's a slack based application doesn't mean you have to get them from slack.
By the way I love the product. One thought: stackoverflow ads? https://www.stackoverflowbusiness.com/advertise
