The Networked City team use Slack. The key features are:
Slack is easy to use on mobile devices, and you can set up email notifications to alert you to conversations.
Sign in at https://networkedcity.slack.com/ if you have an invite, or email firstname.lastname@example.org for an invite for yourself or someone you recommend.
Choose something like firstnamelastname for your username so people can recognise you easily. Add a photo or other image to your profile.
Review the getting started checklist for some introductory videos
While you can use Slack through your web browser, it is better with desktop and mobile apps.
Here's what Slack looks like on the desktop