If you ever have used something like SELECT * FROM table WHERE field IN ('value1','value2','value3','value4') in T-SQL, and wanted to mimic this functionality in the SharePoint SPQuery class, you probably have found out there is no IN statement in the CAML Query language. So what do do? One solution is to use a LinQ [...]
This was unexpected, and not documented at all in the SharePoint SDK. When you create a UserMulti column, you must also specify Mult="TRUE", otherwise it will fall back to a standard (single select) User column type. <Field ID="{12312312-1234-1234-1234-123412341234}" … Group="My Group" Type="UserMulti" Mult="TRUE" …