maid who accompanies bride to her new home, to escort a concubine
媵 centers on a woman who accompanies a bride to her new home, acting as a maid or escort. The concubine sense is a specific historical application of that accompanying role.
The exact historical development of 媵 is uncertain. It visually contains 女 (woman) and a complex right side that may have contributed sound or meaning, but the original component relationships are not clear.
The left side is 女 (woman), and the right side looks like a combination of 并, 大, and 月. Picture a woman (女) carrying a large (大) moon-shaped (月) tray of gifts as she escorts the bride to her new home.
For ヨウ, imagine the maid calling out 'Yo!' to the bride as she escorts her: 'Yo!' -> ヨウ, and the woman (女) leads the way.