My experience with song endings at jams is that the group, playing an unrehearsed song, generally leaves the ending up to whichever player volunteers and "advises" the others that he/she will take the ending by some gesture such as raising their guitar neck, shaking a foot toward the middle of the circle, etc. which is an attention getter. The ending is generally selected based on how the particular song is performed by one or more professional groups. A good ending usually lasts for two or three measures and this lets everyone know what is coming so that all can end together. Familiarity with various endings is a matter of experience, whether you play, learn it from and instructioninal book or someone else at a jam.
With our class protocul set as it as now, practicing an ending would be same as pre-establishing playing order, unless we develop two or more endings, then let the first volunteer take it when we perform.
JJ
