Ask HN: What is a good way to coordinate online for a technical working group?

We have been considering Slack, Whatsapp, Telegram or even plain old mailing lists. Are there any other alternatives? Which ones are the best for a technical working group? Salient features that we are looking for:

- Ability to look at archives

- Multiple threads

- Good desktop support

- Ability to support technical notation

- Support for media and pdf