With Auth0 Webtasks you can quickly create a signup page for your Slack team without worrying about hosting, backends, and devops.
Follow these 3 steps:
npm install -g wt-cli
wt init
wt create https://raw.githubusercontent.com/auth0/webtask-slack-signup/master/slack-invite.js \
--name {your_slack_team}-signup \
--capture \
--secret SLACK_ORG={your_slack_team} \
--secret SLACK_TOKEN={your_slack_admin_token}
Optionally, you can also provide --secret LOGO_URL={url_to_your_logo}
which will display your custom logo on the signup page. It should be square and not less than 100x100px.
Use the resulting URL as your Slack signup page. Enjoy!