To add to what other have said, in your example the "$SGROUPUSER", is just the name of temporary relationship name and the dollar sign is only a convention that does not have any special meaning. You could use whatever name you want, such as the example below where I have used "TEMPNAME" as the relationship name.
userGroupSet = mbo.getMboSet("TEMPNAME", "GROUPUSER", "userid='" + loginuser + "'")
If there were a situation where you needed to get the MboSet again from the mbo variable you could call the following:
userGroupSet2 = mbo.getMboSet("TEMPNAME")
Hope that helps.
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