I want to any user who has account in the domain be able to access some virtual machine. How to set the entitlement to give the right to any user or all users in the domain?
By default, all members in your AD are a member of the "Domain Users" group. Entitle that group to the pool and all users in your domain should be able to see and connect to the pool.
In the "Find User and Group" window, there is Domain drop down. In this drop down, there is "Entire Directory" and other groups, but there is no Domain Users group. Is this something that waht AD was setup?
You're correct, however, you're not just able to search for users but groups as well. In your search field enter Domain Users and you should be able to entitle the whole domain users group to the pool, not just single users.