Who were the 'certain Greeks' who came up to worship at the feast?
The 'certain Greeks' were Gentiles visiting Jerusalem, perhaps on business, who came up to worship at the feast. They were not necessarily proselytes but curiosity seekers, observing the Jewish Passover celebrations and hearing about Jesus of Nazareth and His miracles. They approached Philip, saying they wanted to see Jesus.
Les explains that these certain Greeks were Gentiles among the Jews at the feast. He suggests they may have been visiting Jerusalem on business, and he does not think they were proselytes, but rather curiosity seekers watching the Jews and all their Passover celebrations and activities. They had been hearing about Jesus of Nazareth and the miracles He had been performing, so they desired to see Him. The same Greeks, however many there were, came to Philip, who was of Bethsaida of Galilee, and said, "Sir, we would see Jesus." Les pictures Philip having a hot potato because Jesus had nothing to do with Gentiles for three years, with two exceptions, and Philip did not know how to handle it without embarrassing these people. Philip took the matter to Andrew, and Andrew said they should not take the responsibility on themselves but go in and ask Jesus what He wanted to do. Les imagines Jesus was probably in one of the ante-rooms of the temple area.