Once you and your friends sync a shared library onto your computers, you may add, delete or edit files in the library together. Every change you make to the library will be automatically synced to others' computers.
Sometimes you and your friends may edit the same file at the same time. Your changes may conflict with each other's. In this case, Seafile would keep your version unchanged, while save your friends' versions as "conflict files". A conflict file's name ends with its author's email address, plus the current time, e.g. test.txt (email@example.com 2013-10-01-00-12-24).