Com base nos comentários, você provavelmente desejará ler a página de manual de adduser
.
Em particular:
-G, --groups GROUP1[,GROUP2,...[,GROUPN]]] A list of supplementary groups which the user is also a member of. Each group is separated from the next by a comma, with no intervening whitespace. The groups are subject to the same restrictions as the group given with the -g option. The default is for the user to belong only to the initial group.
Então, no seu script, você pode:
DEFAULT_GROUP=users
exec 3< users.txt
exec 4< passwords.txt
while read iuser <&3 && read ipasswd <&4 ; do
adduser -G $DEFAULT_GROUP $iuser
echo $ipasswd | passwd --stdin $iuser
done