How to create a User field that can contain more than one value

<Field ID="{44DB06B2-DC96-4dd7-AA6A-1A82AE906179}"

         Type="UserMulti"

         Name="Groups"

         DisplayName="Groups"

         ShowInEditForm="FALSE"

         Mult = "TRUE"

         Group="MyGruoup"/>

There are 2 notes:

• Type="UserMulti"
• Mult = "TRUE" - Upper TRUE!