When trying to add users to file share permissions, you get this error:
More than one object is selected. Only one object can be returned. Remove extra objects and press OK.
To fix this, click close, remove all but one username and click ok. You then have to go through and add each user individually to avoid Windows reporting that more than one object is selected.
You get this error because Windows can only apply permissions to one object at a time. This is because the specific permissions need to be set for each object (user account).
A better way to do this, is to create an Active Directory security group with the users you want to add inside it, then add the security group to the folder permissions. In fact, you should use a group to add permissions to a share / folder even if there is only one object to add.