I have been using for some years now the Slack chat system, in several Open Source projects, but also the paid version on the companies I have been working for, or customers.

So we can instant chat in channels or directly to members or group chat up to 5 people.

We can transfer files. (I even use it to transfer from my Linux or Mac to a Windows machine).
We can do video/voice calls.

We lose the history of chats after some time, because it is the free version. Important conversations should be stored on some permanent medium (like email/mailing list). We can apply for the Pro plan for free if with less than 250 members.

In no way this is a replacement for the mailing lists.

This is an invitation link that will expire in 60days (or so):
https://join.slack.com/t/gretlgnuecono-vxh1344/shared_invite/zt-waoqek6r-M2A_lvVbhdYdUV2O8ZyL1w

Hélio