Within Mozilla we utilise groups for category permissions and group messaging.
We import a number of groups from our community directory and restrict certain categories to one or multiple of them. We also use the “Users who register with an email domain that exactly matches one in this list will be automatically added to this group” feature to add staff to a number of groups.
We use group messaging with a couple of groups to create “support ticket style” discussions, by setting a “Custom incoming email address” for the group. This allows anybody to email that email address, with only that person, the group, and anyone invited, able to see the discussion.