Shared folder permissions

This seems too easy, but I can't seem to get it.

Our users log into a domain on our WAN. We have file servers at some
larger locations for file and printer sharing, but at one smaller
location I have two users who desperately need to share files, so I had
the bright idea to use our virus definition server, which already has
file sharing turned on.

I created a C:\Share folder and shared it as Share$. The group
Everyone has full rights to Share. Then I created an Accounting folder
for the two users.

On the Securities tab, I added the two users using their domain names.
I gave them each Full Control to the Accounting folder. I then removed
Everyone -- I think this was my big mistake.

Now, there are no names listed on the security tab. Nobody, not even
the Administrator for this computer, can access this folder. The "Add"
button to add users is greyed out.

Is there any way to add a user back to administer this folder?