The headword -- presumably extracted from the quotation given (though also attested in e.g. Herodotus 7.142.2 and Herodian 1.14.6) -- is imperfect indicative middle, third person plural, of the verb συμβάλλω . See generally LSJ s.v., and cf. sigma 1354, sigma 1355, sigma 1356.
[1] Xenophon, Anabasis 1.1.9, describing how Clearchus, based in the Chersonese, secretly raised a mercenary army for Cyrus in c.402-401. The original text has χρήματα before συνεβάλλοντο αὐτῳ̂ : "they gave him contributions of money".