Well I'll try to explain what I understood...
Here's the function I finally have :
Function ComputerInGroup ($group)
Dim $oGrp
$oGrp = GetObject("WinNT://" + @domain + "/" + $Group + ",group" )
if @error
exit 1
endif
if $oGrp.IsMember("WinNT://" + @domain + "/" + @wksta + "$$" )
$ComputerInGroup=1
else
$ComputerInGroup=0
endif
endfunction

In fact I was wrong with the $domain stuff, I made a syntax error I guess (don't really know) but I realized I didn't need the $domain, since we only have one domain... So I wrote it again using the example you gave me (that's how I saw I wrote doamin instead of domain the first time). Then the function call worked !!!! I'm not sure but does this answer your question ?