well, now
偖 centers on a transitional pause or interjection: a moment to take stock before proceeding, like saying "well" or "now" to shift attention.
偖 combines 亻 (person) with 者, which likely contributes the sound. The exact historical development is uncertain, but the character is used for its sound value in Japanese to represent the interjection さて.
A person 亻 beside 者, like someone pausing to consider who is there: "Well, now, who is this?"
For シャ, imagine a person pausing and saying "Well, now..." while adjusting their shawl: shawl -> シャ, and the pause fits the kanji's meaning.