See this piece by R. Zvi Ryzman (Hebrew).
His bottom line conclusion is:
לכתחילה ודאי ראוי לנהוג כדברי השפת אמת והאמרי בינה שבעל הבית יקנה את המצות לכל מי שיתארח על שולחנו בליל הסדר, מבעוד יום, כדי שיתקיים דין 'לכם' ... אמנם במידה ולא הקנה להם את המצות מבעוד יום, נתבארו לעיל לימודי הזכות שיצאו האורחים ידי חובתם, ואינם צריכים לחזור ולאכול כזית מצה.
Ideally, it is correct to act in accordance with the opinion of the Sefat Emet and the Imrei Binah, in that the host should transfer ownership of the matzot, while it is still day, to those guests who he will be hosting for the Seder, so that the matzot are considered to belong to them ... Nevertheless, if this was not done, we have listed above justifications by which we can understand that the guests have still fulfilled their obligation, and they do not need to eat a further kezayit of [their own] matzah.
Some of those justifications he refers to are:
- Food served to a guest is considered to belong to the guest.
- The matzah is considered to belong to the guest once he puts it in his mouth.
- The guest acquires the matzah by chewing it prior to swallowing it.
- One does not need to fully legally own the matzah; mere permission to eat it is good enough.
- The guest automatically acquires the matzah when he picks it up.
- The host presumably wants the guest to be able to fulfill his obligation, so when he gives the matzah to him we understand that he is actually transferring ownership of the matzah to the guest.