No Subversion repository has any idea how many checkouts there are. So not even the original repository has any idea by how many parties its files are being used (except you use explicit locking, maybe). Each checkout, external or not, can change a file and commit the change. So is it possible at all to tell “borrowing” from “sharing”? What SCM would do that?

Besides, “informing others” of changes is, of course, one of the central functionalities of an SCM. You will be informed about changes in files “borrowed” by others. So what's the difference to “sharing” again?