What you are probably looking for is to change the mode, or permissions of the directory in questions.
If you want to use the gui, it should just be a matter of right clicking on the folder, clicking on properties, and then modifying the check-boxes under the permissions tab.
Or, if you want to be a guru, you can try using the chmod command.
Either way, you can set other user's or group's permissions using chmod by getting the sum of the corresponding permissions for 'owner' 'group and 'other' (you can also opt for setgid mess, but that should be necessary here) where r=4, w=2, and x=1.
Example: You own directory "reply" and you want yourself to be able to read write and execute, people in your groups to be able to read and execute, and others to have no privileges, (rwx r-x ---) you would type:
chmod 750 reply
because 4(read)+2(write)+1(execute)=7 4(read)+0(write)+0(execute)=5 and 0(read)+0(write)+0(execute)=0 (750)
So there you have it. chmod is a damn confusing command, and it has some properties i didn't mention here, so you should review it (god knows i had to so i could write this post)