via registration file "users" namespace:
```YAML
...
namespaces:
users:
- exclusive: true
regex: '.*luke.*'
group_id: '+all_the_lukes:hsdomain'
...
```
This is part of giving App Services their own groups for matching users. With this, ghost users will be given the appeareance that they are in a group and that they have publicised the fact, but _only_ from the perspective of the `get_publicised_groups_for_user` API.
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