Angels can make themselves visible and invisible

Observe the highlighted portion of the following text:

Luke 2:8-14 (ESV) 
8 And in the same region there were shepherds out in the field, keeping watch over their flock by night.
9 And an angel of the Lord appeared to them, and the glory of the Lord shone around them, and they were filled with great fear.
10 And the angel said to them, “Fear not, for behold, I bring you good news of great joy that will be for all the people.
11 For unto you is born this day in the city of David a Savior, who is Christ the Lord.
12 And this will be a sign for you: you will find a baby wrapped in swaddling cloths and lying in a manger.”
13 And suddenly there was with the angel a multitude of the heavenly host praising God and saying,
14 “Glory to God in the highest, and on earth peace among those with whom he is pleased!”

Either the many angels that suddenly appeared with the first angel instantly teleported themselves to this location, or they were there all along and just not visible to the shepherds. Scripture gives us at least one specific instance when angels were present with people and not visible to them.  2 Ki 6.17 records how Elisha's servant could not see the army of Heaven surrounding he and Elisha until his eyes were opened. So it is most likely the many angels were already there with the angel announcing Jesus' birth to the shepherds, but initially kept themselves invisible so the Shepherds could and would pay attention to the single angel making the announcement. When the announcement was heard and understood, the other angels made themselves visible to the shepherds and joined in an acclamation of praise to God.