Adam Clarke on Exodus 19:8

And all the people answered, etc. - The people, having such gracious advantages laid before them, most cheerfully consented to take God for their portion; as he had graciously promised to take them for his people. Thus a covenant was made, the parties being mutually bound to each other.

Moses returned the words - When the people had on their part consented to the covenant, Moses appears to have gone immediately up to the mountain and related to God the success of his mission; for he was now on the mount, as appears from Exo 19:14.