The pair of fields UserDomain and UserName constitute the unique key to each row in the tblADusers table, so you need to link against both. This should establish the link:
INNER JOIN tblADusers ON tblAssets.Userdomain = tblADusers.Userdomain AND tblAssets.Username = tblADusers.Username
and then you add the fields you want from tblADusers.
I'm trying this out on my database and keep being presented with errors. First
Error: Conversion failed when converting the nvarchar value 'exampledomain' to data type bitand, after I filter tblAssets.UserDomain and tblAssets.UserName for NULL and '',
Error: Conversion failed when converting the nvarchar value 'exampleusername' to data type int(where 'exampledomain' and 'exampleusername' represent actual values from my database in the error messages)
Since there are no bit or int data types in the link, I'm at a loss to explain. I tried to link tblADusers against the default Lansweeper report and it worked fine.
SELECT TOP 1000000
tblAssets.AssetID,
tblAssets.AssetName,
tsysAssetTypes.AssetTypename,
tsysAssetTypes.AssetTypeIcon10 As icon,
tblAssets.IPAddress,
tblAssets.Lastseen,
tblAssets.Lasttried,
tblADusers.Name
FROM
tblAssets
INNER JOIN tblAssetCustom ON tblAssets.AssetID = tblAssetCustom.AssetID
INNER JOIN tsysAssetTypes ON tsysAssetTypes.AssetType = tblAssets.Assettype
INNER JOIN tblADusers ON tblAssets.Userdomain = tblADusers.Userdomain AND tblAssets.Username = tblADusers.Username
WHERE
tblAssetCustom.State = 1
I don't have time to troubleshoot right this minute, so I'm going to assume it's just something wonky on my side. You may want to make a copy of your report to try this with, just to be safe, though.