Complex sentence: During the 1960s, an informal community centered on the therapeutic practices of the Institute began to form, and at its peak in the late 1970s, this community had several hundred members (patients and therapists) living on the Upper West Side.
Split into simpler sentences: During the 1960s, an informal community centered on the therapeutic practices of the Institute began to form. At its peak, in the late 1970s, this community had several hundred members (patients and therapists) living on the Upper West Side.