I tried generating a Group History report after a long and tedious battle to install FIM Reporting.
When you get the report, the Attribute Name/Attribute Value pair is something like:
Attribute Name Attribute Value
Explicit Member 7FB2B853-24F0-4498-9534-4E10589723C4
We can explain what Explicit Mmeber means but it is hard to explain why the report dumps the users internal ID and not his name.