Complex sentence: In 2010 the two guitarists, Pontus Hjelm and Johan Olsson left the band however Pontus remained a songwriter for the band, later rejoining as the "session" guitarist and then later again as a permanent member.
Split into simpler sentences: In 2010 the two guitarists, Pontus Hjelm and Johan Olsson, left the band. Pontus, remained a songwriter for the band, however rejoined as the "session" guitarist, then later as a permanent member.