Incompatible group permissions in Linux - Is it a bug?

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http://unix.stackexchange.com – I am on Ubuntu 11.04. I am creating another user and placing an existing user in the group of other user, hoping to write in the home directory of other user. # uname -a Linux vini 2.6.38-11-generic #50-Ubuntu SMP Mon Sep 12 21:18:14 UTC 2011 i686 athlon i386 GNU/Linux # whoami sachin # su root # useradd -m -U foo // create user foo # usermod -a -G foo sachin // add user `sachin' to group `foo' # chmod 770 /home/foo/ # exit # whoami sachin # cd /home/foo/ bash: cd: /home/foo/: Permission denied # groups sachin sachin : sachin foo This is totally weird. Though user sachin (HowTos)