Can a user change groups? Instead of [s]et [u]ser - [s]et [g]roup

[Bill Irwin]

I'm wondering if there is a way for a user to temporarily change
their group.  That is, register a user as a member of more than
one group in the /etc/group file, then have one of the group id
numbers set in /etc/passwd, so that they belong to their main
group when they log in.  If they later want to access some files
that are not owned by them, nor their group, but are of a group
that they are also a member of - they would change their group,
work with the files, then return to their original group.

The concept is exactly the way the su facility works.  Could it
be called "sg" for "set group"?
-- 
Bill Irwin    -       The Westrheim Group     -    Vancouver, BC, Canada
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