I'm looking for a NAS to simply: Again it's around 20 users and computers. Should the 220+ handle this well or should I be looking at something faster?
You can do this by selecting [Access Control] [ Domain Users] or [Domain Groups]. Click on icon will connect to the AD server and update the user/group list.
Correct install the Synology Directory Server package from the package center and set up your Windows Domain. It's an excellent choice for small businesses.