try this:
Select Top 1000000 tblCPlogoninfo.Username, tblCPlogoninfo.Domain,
IsNull(tblADusers.Displayname, '') As Fullname,
Count(tblCPlogoninfo.Computername) As total
From tblCPlogoninfo Left Join
tblADusers On tblADusers.Username = tblCPlogoninfo.Username And
tblADusers.Userdomain = tblCPlogoninfo.Domain
Where tblCPlogoninfo.logontime > GetDate() - 30
Group By tblCPlogoninfo.Username, tblCPlogoninfo.Domain,
IsNull(tblADusers.Displayname, '')
Order By Count(tblCPlogoninfo.Computername) Desc
Total = amount of computers