Great question. It's always best to log in using the direct number if the user has one. Otherwise the softphone will not have all the same functions and the user could miss calls to their direct number.
Because of the nature of Shared Lines being a group and not an extension, you will not be able to take calls on the softphone.
The system may let you log in because you would be using a valid phone number and password, but Shared Lines is not meant to be used with a softphone so it will not function as a softphone would on a User's extension.