Amphidamas and his brother Kepheus the sons of Aleus and Kleubule came from Arkadia to join the crew of the Argo. This of course is according to Hyginus in Fabulae.
"Amphidamas and Cepheus, sons of Aleus and Cleobule, from Arcadia."
- Hyginus, Fabulae, 14
"Moreover from Arcadia came Amphidamas and Cepheus, who inhabited Tegea and the allotment of Apheidas"
- Apollonius of Rhodes, Argonautica 1.61