This is not a complete solution, as i do not have your data and schema. But what i think you need is pivot Faceting http://wiki.apache.org/solr/SimpleFacetParameters#Pivot_.28ie_Decision_Tree.29_Faceting
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So you need to do something like this (again , you need to adjust this to make it work for you)
http://ip:port/solr/collection1/select?q=*:*&rows=0&facet=true&facet.pivot=Username,ServerName