Usage: 2 forms in 3 verses
Form Translated As     In Verse(s)
προσχωρήσαι to join LXX.Jer.21.9
προσεχώρησαν joined LXX.1Chr.12.19
some joined with LXX.1Chr.12.20

Liddell-Scott-Jones

προσχωρέω (proschōreō) to join with G:V

προσχωρέω-έω,

future -ήσω[Refs 5th c.BC+]; also -ήσομαι [Refs 5th c.BC+]:—go to, approach, with dative, προσεχώρεον πλησιαιτέρω τὸ στρατόπεδον τῷ στρατοπέδῳ [Refs 5th c.BC+]

__II come or go over to, join, [τῷ Ἑλληνικῷ ἔθνεϊ] [Refs 5th c.BC+]; πρὸς ὁποῖον βίον ἄλλον.. προσχωρέωήσεται to what other sort of life he will give himself up, [Refs 5th c.BC+]
__II.2 side with, support, οὐκ ἐθέλει οὐδὲ θεὸς προσχωρέωέειν πρὸς τὰς ἀνθρωπηΐας γνώμας [Refs 5th c.BC+]; comply with, τοῦδε π. λόγοις [Refs 5th c.BC+]
__II.3 approach, i.e. agree with, be like, τὰ νόμαια Θρήϊξι προσκεχωρήκασι [Refs 5th c.BC+]
__II.4 of funds, to be applied, εἰς συνωνὴν πυροῦ [Refs 3rd c.AD+]
__II.5 μὴ προσχωρέωηθέντος written in error for μήποτε χωρισθέντος in [Refs 2nd c.BC+]