If you Dim the object variable in the function it will remain local to the function. Otherwise, you can still access the object.

Demonstration...

code:
Break On

GetUserName(@WKSTA) ?

For Each $objComputer in $colComputer
	$objComputer.UserName ?
Next

Function GetUserName($sComputer)
	;Dim $colComputer
	$colComputer = GetObject("winmgmts:{impersonationLevel=impersonate}!\\" + $sComputer + 
		"\root\cimv2").ExecQuery("Select * from Win32_ComputerSystem")
	For Each $objComputer in $colComputer
		$GetUserName = $objComputer.UserName
	Next
EndFunction