Complex sentence: Jeffes and cellist co-founder Helen Liebmann were the only core members throughout its life; a number of other musicians appeared on the PCO's six studio albums according to the requirements of particular pieces.
Split into simpler sentences: Jeffes and cellist co-founder Helen Liebmann were core members throughout its life. A number of other musicians joined the collective as the band grew and developed, and many of these appear on the PCO's six studio albums as required by the instrumentation of particular pieces.