Nemi can be reached by bus from San Gennaro, where the agreement with Trenitalia allows the pilgrims that travel with the EAVF Credential to benefit from favourable rates (10% decrease) on the price of regional train tickets, in the authorised segments (for more information, see the regulation).
By buying the train ticket for Nemi from Trenitalia, the bus ticket from San Gennaro to Nemi is already included in the price, as specified here.