Reading Ανάβασις Ξενοφόντος. Here Xenophon is talking to Χειρίσοφος about what to do about some lurking non-Greeks (i.e. Persian adversaries) because the Greeks are in a bit of a tight spot.
ἀλλὰ δίδωμι σοι, ἔφη ὁ Χειρίσοφος, ὁπότερον βούλει ἑλέσθαι. εἰπὼν ὁ Ξενοφῶν ὅτι νεώτερος ἐστιν αἱρεῖται πορεύεσθαι, κελεύει δέ οἱ συμπέμψαι ἀπὸ τοῦ στόματος ἄνδρας.
It's the end of the second sentence I'm struggling with. The parallel translation has this:
Then Xenophon, with the remark that he was the younger, elected to go, but he urged Cheirisophus to send with him some troops from the front.
It's "οἱ συμπέμψαι" that I don't really understand. In fact I can't work out whether "συμπέμψαι" is aorist active infinitive or 3s aorist active optative. I think it's probably the former, so perhaps another variation of the "accusative + infinitive" construction which seems to be used often in Ancient Greek.
So this would be "he urges ... men ... from the front (of the army) to be sent with (him)"... but in that case what is "οἱ" doing there? This is the nominative plural article, obviously. Why wouldn't it be the accusative plural article?
(I'm assuming in the above that the article can indeed function as a relative pronoun, and believe I've seen this before a few times already...)