Only the GitHub/Slack admin needs to create a GitNotifier account and complete the installation through the UI. Once installed, any team members can join from slack with one click.
Install GitNotifier on your GitHub account or organization from the dashboard.If you are not a GitHub organization admin, GitHub will let you request the installation instead. A GitHub organization admin can then approve that request directly in GitHub.GitHub documents this flow here: Requesting a GitHub App from your organization owner. When you submit the request, GitHub notifies your organization owner so they can review and approve the installation.