Falir is definitely the way I’d go too, but it relies on the group being the primary group of the user-- which fails as soon as the user plays with those settings.
Another alternative for this would be to automatically assign a badge to users in the group. This means the staff can just click the name and see from the profile which groups the person is in – not as immediate as flair, but less likely to suffer from user interferance(users can’t remove badges as far as I’m aware).
SQL query for the badge, where in this example “discourse-admins” is the Group slug.
select user_id, created_at granted_at, NULL post_id
from group_users
where group_id = (
select g.id from groups g where g.name = 'discourse-admins'
)
And these are the settings
