Express"with each other" or "amongst themselve" I'd like to express the idea that the members of a group perform a multi-party action, e.g., conversing, amongst themselves.　E.g., "My co-workers use English when they talk amongst themselves".

The best I can do right now is:

我同事互相交谈用英文。

But I imagine there's a lot of work to be done here, both in expressing the notion of "amongst themselves" and translating the "when X, Y" subordinate structure.
"My co-workers use English when they talk amongst themselves".
我的同事们之间用英语聊天儿。