I’m the founder of the MODO Slack User Group and Slack has proven to be a lot more clean, efficient and functional for the types of group conversations we need than Telegram or Skype.
There are also open source alternatives to Slack now: http://www.wired.com/2016/03/open-source-devs-racing-build-better-versions-slack/
I haven’t tried any of those myself but Slack is extremely solid for work group conversations including features like intuitive and visually pleasing embedding of image and video content, file sharing via Google Drive, Dropbox and many other cloud storage options, code formatting and colour highlighting and many other useful aspects.
Also available in-browser and as Windows, OSX, iOS, Android, Linux and even Windows Phone apps.
I like Telegram and use it often but it gets pretty messy and confusing with more than a couple of people. Slack (and I assume its FOSS competitors) stays very clean and manageable by comparison.
Here are two screenshots from our MODO User Group:
Just for your consideration.