• Slack(Paid, proprietary, generous free access): If you just want to just use group mesaaging for small group upto couple of thousands of members
• Discord(Free, proprietary): Great choice if you want to create a community of upto 100k. Members can maintain their anonymity in this one.
• Mattermost(Free, open-source, hosted paid alt): Great choice of you want to self-host and keep the control of data. Also a great starting point if you want to customize the code and make your own chat app.