[–] 1355678? 0 points 1 point 1 point (+1|-0) ago
Do they use a active directory? If so you can have configuration to either provide a list of users with access (easier for you as you can easily query the current user logged in) or the group they should belong to to have access (a little trickier but in 3.5 you can use System.DirectoryServices.AccountManagement).
[–] jewdai 0 points 2 points 2 points (+2|-0) ago
IIS supports Windows Authentication. They'll still have to enter in their username and password but it will be tied to their active directory account.