You could create groups A and B, and just use private messaging to one or more these groups to achieve what you are subscribing.
Doing so is probably the closest way to reproduce the communication model you are used to with email groups.
But is that really what you want to do?
Are messages to group A not supposed to be shared with group B due to privacy reasons? Or is it simply because it would be noise if they were always cross-posted?
One of the great benefits of a forum or message-board model like Discourse is that topics can be open to all while their relevance can be indicated by the category they are in. This reduces the mental overhead of choosing which groups to CC and allows more open participation and cross-pollination across teams.
If its the latter, I would consider creating categories instead of groups. That way they are visible to both groups, but organized in such a way that group A can more easily pay attention to messages in category A (or watch them, etc).
Categories can be access-controlled on a group basis as well, so there are additional things you could try with categories and groups if you have more specific privacy requirements.
For example:
- category A1: mainly of interest to group A, but visible to all
- category A2: private to group A
- category B1: mainly of interest to group B, but visible to all
- category B2: private to group B
- category C1: mainly of interest to group C, but visible to all
- category C2: private to group C
- category AB: private to groups A and B
Also, coincidentally I was planning on posting this other topic about Discourse and Email Lists today.