Handing out House Keys to Preclude Yichud
A primary condition of yichud (halachically proscribed seclusion of male and females) is lack of free access or oversight. Hence, a private home that has pesach pasuach l’reshus harabim (a door that is open to a public domain) would avert the issur (prohibition) of yichud during the day and early evening hours, at a time when there is plenty of pedestrian traffic.
Some later acharonim (latter-day halachic authorities) extend the exemption of pesach pasuach l’reshus harabim to private homes with regular access by others, such as when the house keys are in the hands of outsiders. However, in order to have the same status as pesach pasuach l’reshus harabim, the following conditions must be met:
There must be access for a rabim, a minimum of three people who live locally and would conceivably visit during this time.
It's daytime or early night hours when people are around, following the same restriction as pesach pasuach.
There is no surveillance system in place whereby those secluded in the house can receive advance warning that someone is coming.
The man and woman don’t have a companionable relationship, if any at all. On the other hand, if they are classified as libo gas bah (they are on friendly terms), according to many poskim, the exemption of pesach pasuach—and certainly key-holders—would not apply.
It is a shaas hadechak (there are extenuating—urgent—circumstances) and is not relied on as a regular solution.
It is offset by other factors that ameliorate the issue of yichud (such as specific ratios of women to men, the complex details of which need further study).
Aside from the exemption of pesach pasuach—which includes, according to some, a house-key heter, too—there is an additional principle of shmirah (supervision) called yotzei v’nichnas (a person comes and goes). In order for the third party to be considered “coming and going” they must pop in often. In addition, they must stagger their visits so that there isn’t a long stretch between appearances, and at times they return quite soon after departing.
(See Halachah #356 and Halachah #658 for additional information regarding shmirah when yichud is an issue, and Halachah #55 for other exemptions. For another application of yotzei v’nichnas unrelated to yichud, see Halachah #212.)