Complex sentence: At first he was reluctant to book the Beatles because the band did not have a single released in the US at the time, but at the behest of a friend, legendary impresario Sid Bernstein, Sullivan signed the group.
Split into simpler sentences: At first he was reluctant to book the Beatles because the band didn't have a single released in the US at the time. But at the behest of a friend, legendary impresario Sid Bernstein, Sullivan signed the group.