I know this is an old thread, but I had to do it recently, and it is still relevant.
Since we are talking about * nix permissions, one tedious but thorough way to approach this is to look at the membership that the ID has in the system:
t
# assuming josh is a member of group "grpname" find / -user josh -perm -100
Note that there may be some coincidence for files owned by josh, but also belongs to the "grpname" group. Grade | uniq will easily filter them.
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