We need to acknowledge at the start that Mark's primary goal in describing the trial of Jesus is to show that he was unjustly executed. From the beginning while he was operating in Galilee, various factions conspired to eliminate Jesus. Mark 3:6 (ESV):
The Pharisees went out and immediately held counsel with the Herodians against him, how to destroy him.
The intrigue kicks into high gear in Mark 14:1-2 (ESV):
It was now two days before the Passover and the Feast of Unleavened Bread. And the chief priests and the scribes were seeking how to arrest him by stealth and kill him, for they said, “Not during the feast, lest there be an uproar from the people.”
Possibly the "chief priests" should be read as the Sadducees and the "scribes" as the Pharisees based on their traditional roles. Or it could just mean the priests in charge of sacrifices and feasts at the Temple and the scholars who studied and copied the Torah and the Tanakh. Their decision is to wait for the people who flooded Jerusalem for the Passover to leave before going after Jesus. They change plans soon, however, when an opportunity drops out of nowhere. Mark 14:10-11 (ESV):
Then Judas Iscariot, who was one of the twelve, went to the chief priests in order to betray him to them. And when they heard it, they were glad and promised to give him money. And he sought an opportunity to betray him.
In the middle of the night, Judas and a gang arrest Jesus according to Mark 14:43 (ESV):
And immediately, while he was still speaking, Judas came, one of the twelve, and with him a crowd with swords and clubs, from the chief priests and the scribes and the elders.
A reasonable reading of this passage is that the leaders sent their servants along with Judas, but it could have been temple guards. The term elders here might be specifically the Sanhedrin or just a general group of respected citizens. Given that Judas sparked a change of plans, it reads as if the group sent was a disjointed set of officials who happened to hear of the new conspiracy.
The first stop is the house of the chief priest where everyone gathers. Mark 14:53-55 (ESV):
And they led Jesus to the high priest. And all the chief priests and the elders and the scribes came together. And Peter had followed him at a distance, right into the courtyard of the high priest. And he was sitting with the guards and warming himself at the fire. Now the chief priests and the whole council were seeking testimony against Jesus to put him to death, but they found none.
It's hard not to read this group as the Greater Sanhedrin of 70 or 71 members since the text makes a point of the council being "whole". (The Greek word for "whole" is holos: "In NET: whole 47, all 32, entire 8, throughout 8, completely 3, All 2, a single piece 1, around 1, in 1, full 1".) From what I've read, the Sanhedrin met every day and since it was a feast day and the council had responsibilities to fulfill for the Passover, we can assume that rounding them all up would not have been difficult. However, the high priest's house would not have been their usual meeting place.
The Sanhedrin traced its roots back to the Gerusia which was established to assist Moses. Numbers 11:16 (ESV):
Then the LORD said to Moses, “Gather for me seventy men of the elders of Israel, whom you know to be the elders of the people and officers over them, and bring them to the tent of meeting, and let them take their stand there with you.
(Mark mentions testimony from false, confused and contradictory witnesses prior to Jesus' own confession. It seems clear the accusers hadn't had time to get their stories straight, which confirms the idea that the conspirators' plans changed at the last minute.)
The complicating factor is a second meeting recorded in Mark 15:1 (ESV):
And as soon as it was morning, the chief priests held a consultation with the elders and scribes and the whole council. And they bound Jesus and led him away and delivered him over to Pilate.
One way to read this is that the council was still gathering during the time of the first meeting and only when all had arrived did they send Jesus to Pilate. Another reading, that seems more likely, is that after the council condemned Jesus to death, they took a short break until morning. The purpose of the wait might have been to legitimize their decision, which was originally made in the dead of night, or so that they wouldn't disturb Pilate and his household in their sleep.
Without any evidence to suggest that the council was other than the Great Sanhedrin, we have to assume that Jesus was condemned to death by what we might call the "Supreme Court" of the Jews.