Multi-user IMs for intra-team communication

I have an extended team that works well together but are slow to reach out between sites to pick brains and throw out ideas. What I’d like to do is take the quick informal give and take of IM but have it work for groups not individuals. An immediate give and take would be ideal which is why IM occurred to me rather than chat groups.

Is there such a thing? Has anyone here any experience with an idea like this?

Why wouldn’t the chat function of just about any IM app work? I guess I don’t get your distinction between IM and “chat groups.”