If I understand what you want then you can set up the base directory of your jail the same way that I set up my /home directory.
Code:
/jail root:users 710 (rwx--x---)
|-/etc root:users 710
|-/lib root:users 710
|-/usr root:users 710
|-/user01 user01:user01 770
|-/user02 user02:user02 770
|-/user03 user03:user03 770
The idea is that on the /jail directory the 'others' permission setting is zero, the group permission setting is execute only, and the owner permission is read, write, and execute. Then the owner is root and the group is users.
In my example I have the same permissions and ownership on the jailed system directories. You may need to loosen that depending on what how your applications work.
You can see that I have set the ownership of the users' home directories to the specific user as the owner and as the group. That is because I create a group for each new user. It comes in very handy when setting file permissions.
In your case you may have to have the directories owned by the root user and the ftp group. Or you may have to have the /jail directory owned by root:users but have 711 permissions. Hopefully you get the idea and you can play with it a bit.