Not all of the fields in the User object are searchable out of the box. The recommended way to make these fields searchable is to:
Create a hook plugin that overrides the User service to index the required fields
Create a hook plugin that overrides the edit user Struts action to index the required fields
Create an Ext plugin that extends the User indexer to index the required fields
Create a hook plugin that implements an indexer post processor to index the required fields
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