NMGroups is a number field
If that is true, then it can only hold one number. So that would mean your INSERT should not work, either.
It must be a text
field of some kind or it could never hold more than one number.
DEBUG DEBUG DEBUG
NMGroups = Request.Form("NMGroups")
strSQL = "UPDATE NewsletterMembers set NMGroups = '" & NMGroups & "' WHERE NMID =" & NMID
Response.Write "<hr/>DEBUG SQL: " & strSQL & "<hr/>" & vbNewLine
If you put in the debug code
shown there and run the page, what does the debug output to the screen?