Go, assemble the elders of Israel and say to them, 'The LORD, the God of your fathers--the God of Abraham, Isaac and Jacob--appeared to me and said: I have watched over you and have seen what has been done to you in Egypt.- Exodus 3:16 (NIV)
Moses was to gather the people to garner the buy-in. The burning bush encounter wouldn't be enough to convince the children of Israel that God had spoken through the former prince of Egypt who now returned as a self-proclaimed prophet of God for their deliverance. God told Moses what He planned to do. God told Moses how He planned to use him. It still did not register as enough for Moses. He even offered Moses the evidence of signs if the people would not hear his voice.
God offered Moses His divine guidance and support. God offered Moses signs before his very own eyes. God gave Moses everything he needed. How did Moses respond?
But he said, “Oh, my Lord, please send someone else.”- Ex. 4:16 (ESV)
God answered all of Moses' excuses, even his request for someone else to take on the task. God asks about Aaron the Levite, the brother of Moses. He tells Moses that He will use Aaron along with Moses to speak to the people and reveal what God's plan for them to them. They would even work together with the elders to take their request before the king of Egypt.
Moses got more than he asked for with this task. God did not relieve him of the responsibility of the task. He offered him relief for his fear and reassurance through Aaron his brother being his mouthpiece. The task was still in the hands of Moses. He was still to take on the task to lead God's people to deliverance.
28 And Moses told Aaron all the words of the Lord with which he had sent him to speak, and all the signs that he had commanded him to do. 29 Then Moses and Aaron went and gathered together all the elders of the people of Israel.- Ex. 4: 28-29 (ESV)