Permissions and Groups on a New Server

I set up a new instance of Ubuntu 12.04. This has, of course, the root user. I have apache running and it has it's own user and group and I created a single ftp user as well (that has it's own user and group). I also created a new group that I thought I had set up to contain the apache and the ftp user to upload/create/manage files.

So now, in my web root and with regard to users and groups, I have something that looks like this:

-rw-r--r--  1 root     www-pub    22 Jul 30 06:21 index.html
drwxr-sr-x  5 www-data www-data 4096 Jul 30 06:26 test
drwxr-sr-x  8 ftpacct  www-data 4096 Jul 30 06:03 stuff

index.html was created by root (via ssh), test dir was created by apache and stuff dir was created by the ftp user.

(Additionally, www-data and ftpacct both belong to the group www-pub.)

My problem: if a file or dir is owned by apache (www-data) it is not modifiable by the ftp user (ftpacct) and vice versa. I can give the element 775 permissions and THEN it is editable as all users are in the same group but this seems insecure (maybe it isn't? I'm of the impression that anything more than 755 isn't a good idea).

So what is the solution? How can I make it so that files uploaded by ftpacct can be modified by user www-data and so that files created by that user can be modified by ftpacct? Or should I even be thinking like this?

Can I do this or is this permissions setup just not the best way to do things?