Slack is a more structured chat platform, it allows users to reply in threads and to have different "channels" (like topics). Have a browse of the different channels and get involved (not all channels are automatically added).
Slack can be used through the web browser, desktop app, or mobile app. you can set the notifications as you wish so you don't need to be notified of every conversation but for example you can limit it to replies to your comments, or when key words are said (eg "CAD" or "laser" etc).
If you're a paying member and on Slack make sure you make it known so you can be added to the Members channel.
The decision to use Slack is historic with not many alternatives available at the time of incorporating Teesside Hackspace, (IRC was decided against). In the spirit of using Open Source software we are now running a Matrix server, this synchronises with Slack so either platform can be used. Our Matrix server is still undergoing testing and will be undergoing changes shortly, Slack is currently still the messaging platform of choice.