Presumption: You know the other users' passwords.
1. Open VSS Explorer
2. Choose File > Open SourceSafe Database
3. Change the Username (the user who has checked out the files)
4. Once you change the Username, "Open" button will be enabled
5. Click Open and enter the user's credentials
6. Now you should be able checkin the files
Sometimes I do my development stuff in my laptop and sometimes in my desktop, and I share code using VSS...this approach had helped me many times as I know both the systems' credentials. I don't have any idea how to do this without knowing the other users' credentials.