In the sentence "In spite of the large crowds, we enjoyed the exhibit," in spite of is the compound preposition.
Compound prepositions are those prepositions which are formed by prefixing the preposition to a noun, an adjective or an adverb. There exists two-word prepositions and three-word prepositions . The last word of a compound preposition is always one of the simple prepositions. In this paticular case the simple preposition is OF.