In Germany and Austria it is customary in mostly working and middle class "Kneipen" (the traditional bars) to knock on the table when joining or leaving a group of friends or colleagues already seated.

This custom is a useful time saver as it avoids having to shake the hand of each person when you arrive or leave.

In this case there are only one or two taps on the table, rather than the dozen or so (or more) used as a form of applauding in the academic environement, after a speech or a good lecture.