should I open up my Slack community or still have ppl sign up for newsletter to get access?

Maybe just ask for the email with a checkbox to also subscribe to the newsletter?

It's not too much to ask for an email to gain access to a slack community. Especially in the age where anonymous email forwarding tools exist, such as

I wouldn't worry too much either way. If your concept is so popular that people are piling onto your Slack, then that's a good problem to have. You're much more likely to be overwhelmed by apathy.