I am working with students using Digital Editions and have heard of 2 main issues/error messages. The first says "unable to download, Error getting License" (the book has already been fulfilled by another user) and the second issue is "Adobe Digital Editions 2.0 wants to use the "Login" keychain" and it asks for a keychain password.
Each book 'instance' can only be used on a single Adobe ID.
The .acsm file is a token for the book, and is not initially specific to any particular ID.
Once it has been used to download a DRM .epub file, both the .acsm and the .epub are locked to that ID.
Attempts to use the same .acsm for a user on a different id will quite correctly give a '(the book has already been fulfilled by another user)' message.
The ID can be shared by up to 6 devices at a time.
Once used, the .acsm/.epub can never be transferred to a different ID.
I am not at all sure about the login keychain.