group members can't modify files even though 775 set

[Seamus Abshere]

Dear freebsd-questions,

I want all members of the "webmasters" group to be able to modify my web 
documents directory /www.

Here's my user, a webmaster:
> $id sabshere
> uid=1000(sabshere) gid=0(wheel) groups=0(wheel), 1002(webmasters)
and, just in case, here's root:
> $id root
> uid=0(root) gid=0(wheel) groups=0(wheel), [...], 1002(webmasters)

Here's my directory:
> $ls -l
> -rwxrwxr-x  1 root  webmasters  3714 Dec 19 16:49 index.html

But this happens when I am logged in as sabshere:
> $echo "whatev" > index.html
> -bash: index.html: cannot overwrite existing file

I thought this had to do with the "sticky bit," but "chmod -R -t /www" 
doesn't do anything.

Thanks for any insight you can give,

Seamus