determiner , pronoun ~ of (we saw ~ of them)
USAGE NOTE: The use of the preposition of is necessary when a personal pronoun follows. When a noun follows, of is omitted--the dean saw ~ students ('the dean saw every student'). To express limited meaning, of the or the (very common in BE ) can be inserted
--the dean saw ~ students who had received poor grades = the dean saw ~ (of) the students who had received poor grades.