companion
儕 centers on the idea of a companion or peer: someone who stands alongside you as an equal.
儕 combines 亻 (person) with 齊, which likely contributes both sound and the idea of equality or evenness, suggesting people of equal standing.
A person 亻 beside 齊, a shape associated with evenness or alignment, suggests someone who is your equal: a companion.
For セイ, imagine a companion saying 'say' something to you: 'say' -> セイ, and your companion speaks.