Basically, you'll have to use a subquery in the where clause. This subquery will contain the assetID of all asset where the accout testuser is found. With the where clause, you set the condition that the report must show all results except when the asset ID is identical to an assetID where the account testuser was identified.
So basically, you can create a report that shows all the information about the asset you want, and put this in the where clause:
Where tblAssets.AssetID NOT IN (Select tblAssets.AssetID
From tblAssets
Inner Join tblUsersInGroup On tblAssets.AssetID = tblUsersInGroup.AssetID
Where tblUsersInGroup.Username = 'testaccount' And tblUsersInGroup.Groupname =
'users')