"Should we now proceed? If you are in a situation where a group is gathering to do something (go out for lunch, start a meeting, etc.), "Shall we?" would mean something like "Are we now all ready and is it now time to proceed with what we are here for?". (Or more simply, "Should we now proceed?", but that explains "shall" in terms of "should".)

When you shall, and need to make sure others shall as well.