IIS7.5 - about app pool ID's and folder read/write access

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http://serverfault.com – I did some searching and it looks like for each app pool, there should be an account created called IIS APPPOOL\AppPoolName - however i can see no such account when i try to modify the permissions on a folder to give that app write access. The closest I have found is the IIS_IUSRS group. Now, if i go into that group and look at the members, i see several IIS APPPOOL\PoolName members. But where are these members coming from? Why don't they show up under the users? And why can't i add a specific one to a folder? It doesn't make sense to me to add the IIS_IUSRS group to a folder since they gives (HowTos)