Just watch Megumi's Ribbon LD.  In the second song "A Late Comer", one scene
showing Megumi singing in a studio wearing pajama.  3 background vocal girls
can also be seen.  At the last part of the LD comes the end credits, showing
the names of the background vocals.  One of them is Mitsuishi Kotono.

I did not recognise her so I rewatch the song 3 times to identify her.
She is quite different in looks then.  (FYI, Ribbon LD is published in 1990).

Ja...CuSO4