My newly added user cannot write to folder

Suppose I installed ubuntu with the user usr1 then I added usr2. I then create a group grp1 where this 2 users are added. A folder is owned by usr1 and has group grp1. But when I login as usr2 I cannot write. The folder has permissions 755...

usr2@kmserver:/media/data/www$ groups usr1
usr1 : grp1 ...
usr2@kmserver:/media/data/www$ groups usr2
usr2 : grp1 ...
usr2@kmserver:/media/data$ ls -l
total 20
drwx------ 2 root root 16384 2011-05-12 10:53 lost+found
drwxrwxr-x 2 grp1   grp1    4096 2011-05-12 15:15 www

Whats wrong?

Solution 1:

"The folder has permissions 755"

Change that to 775.