Orochimaru got his hands sealed when he was fighting against Hiruzen Sarutobi. He got his hands back much later during the war. But when he was fighting 4 tails Naruto at the beginning of Shippuden, he summoned three Rashomon. At that time he still didn't have his hands, so how did he manage to do that?
After Hiruzen used the Reaper Death Seal, Orochimaru arms were immobilized and was unable to do anything with them (ie, perform justu). As a result, Orochimaru was forced to find a new host for his soul due to his current host's condition progressively got worse.
Transferring his soul into a new host cured the physical ailments of Orochimaru's conditions. While his soul was still damaged spiritually, he could at least perform justus that did not require hand seals.
Though the act undid most of the damage caused by the Dead Demon Consuming Seal, it still left Orochimaru incapable of using techniques requiring hand seals.
(emphasis my own)
Summoning justu does not require hand seals. Instead, summoning only requires a blood sacrifice and summoning seal.