Primary and Foreign Key?

I have the following related tables:

PEOPLE
--------------
peopleid pkey,
name,
etc


GROUPS
-------------
groupid pkey,
description,
etc


PEOPLEGROUPS
-------------------
peopleid pkey/fkey,
groupid pkey/fkey


What is the CORRECT way (in Postgres) to define the PEOPLEGROUPS table so
that it has both the double primary key AND still acts as a foreign key for
people.peopleid and groups.groupid? Can i specify both or is this not
necessary? Any suggestions would be appreciated.

Thanks!

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