Complex sentence: The label worked closely with Higher Intelligence Agency and Rockers Hi-Fi, and was closely involved in the Oscillate club, which was to host many of the early performances by ambient dub's pioneers.
Split into simpler sentences: The label worked closely with Higher Intelligence Agency and Rockers Hi-Fi. Concurrently, The Higher Intelligence Agency set up and were closely involved in the Oscillate club, Which was to host many of the early performances by ambient dub's pioneers.