When he saw Peter and John about to enter, he asked them for some money. 

see可接復(fù)合賓語。賓補可以由 as 短語、動詞不定式、名詞、動名詞、形容詞（短語）、副詞、介詞短語或現(xiàn)在分詞, 過去分詞充當(dāng)。接動詞不定式作補足語時,不帶 to, 但如果用于被動結(jié)構(gòu)則不能省略 to。

紅色部分為“形容詞短語”（即將進入），作賓補。

《柯林斯詞典》明確界定，在 be about to do...中，about 是形容詞。

ADJ If you are about to do something, you are going to do it very soon. If something is about to happen, it will happen very soon. 就要…的 [v-link ADJ to-inf]