group '545' does not exist ?

I am probably missing something, but this seems bizarre (to me)

$ id
uid=197608(Marco) gid=197121(NESSUNO) groups=197121(NESSUNO),114(Account 
locale e membro del gruppo Administrators),544(Administrators),547(Power 
Users),545(Users),4(INTERACTIVE),66049(ACCESSO CONSOLE),11(Authenticated 
Users),15(Questa organizzazione),113(Account 
locale),4095(CurrentSession),66048(LOCALE),262154(Autenticazione 
NTLM),405504(Livello obbligatorio alto)


$ newgrp 545 touch prova2
newgrp: group '545' does not exist

$ uname -svr
CYGWIN_NT-10.0-26200 3.6.5-1.x86_64 2025-10-09 17:21 UTC


what I am missing ?

Regards
Marco

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