Well you were suppose to find it out randomly while doing 100 sex quest with her gradually but here's how. You need to have sex with her at least 12 times to get that option.
Dahlia's content is not completely done but you can make the ring affect her by having sex with her in some other places. One is in Sakurai house. Try banishing ghost to see for yourself. After you had sex with her enough times she will become her lewd self again.
Other assistants are not in the game yet.