You can make a nice url for the Slack invites by going into your heroku dashboard under the "Domain" settings, put in a url that you want (I might suggest something like slack.steemit.com) make a CNAME record with your webhost pointing the newly chosen subdomain url for slack to http://steem.herokuapp.com and you should be all set. No more sharing these ugly links to heroku. Now you can have something official :)
If you need documentation about the CNAME records you can read it here: https://devcenter.heroku.com/articles/custom-domains