Complex sentence: MacGregor's party continued east and soon contacted elements of the main revolutionary army, which helped them the rest of the way to Barcelona, where they arrived on 20 August 1816 after 34 days' march.
Split into simpler sentences: MacGregor's party was helped the rest of the way east to Barcelona by elements of the main revolutionary army. They arrived on 20 August 1816, after 34 days' march.