Right now the reference group input (in both MWReferenceDialog and MWReferenceListDialog) is a plain text input box (with a placeholder).
Instead, it should be a drop-down that suggests existing groups, as well as giving the ability to write in a new value, aka a ComboBox.
The default value should still have a placeholder, I think.
[Blocked on OOjs UI having a ComboBox widget.]
Version: unspecified
Severity: enhancement