Sorry for bumping an old topic, but this topic was helpfully suggested to me when I started composing a new topic with the same question. I’m surprised there hasn’t been more demand for this as a feature. It seems that it could be accomplished by expanding on the options that are shown in a group’s “Who can message this group?” dropdown:
I’m wondering if reluctance to implement this is just due to potential UI issues with having to show a huge list of options in the dropdown. If that’s the case, maybe a multi-select input, similar to what’s used for tagging topics could be used for the setting instead of the current single-select input. That way, every possible option of who can message the group wouldn’t need to be listed.
It’s more likely that reluctance is due to lack of demand for the feature. My hypothetical use case would be to grant permission to message a “customer support” group to members of a “customers” group as a perk for making a purchase on an online store. This could even be useful on Meta if there was ever a desire to force all customer support to be handled directly through the forum.