Not totally clear what you're asking. Would the slack group be just for job seekers, or would it have hiring staff in there too? Would it be a separate slack server per company, or literally one server for everyone?
I was thinking one server and just for job seekers. Like a support group for people looking for tech jobs.
Maybe there could be channels per role/level, like #backend-senior, #ml-junior. So you could discuss hiring process and get tips from people looking for similar roles.