Can I lock the event with a PIN?

Yes, and it is on by default: when you create the event you set a PIN of 4 to 6 digits, separate from the event code, and a guest has to enter it once before the gallery opens.

The PIN is the answer to the code being printed on every table: the event code gets a guest to the door, the PIN lets the invited ones in. Tell guests the PIN on the day, on the sign or from the microphone, rather than in an invitation that gets forwarded. They type it once, and their browser remembers it for a week, so nobody enters it again that evening.

You can switch the PIN off before creating the event if you would rather guests only need the code; the creator then tells you that anyone with the link can open the event. Changing or removing the PIN afterwards is done in the dashboard, which you open with a one-time code sent to your email; the host link in the confirmation email is not enough for that.

An event created from a guest page with just an email address gets a random PIN, which is in the confirmation email. The PIN keeps strangers out; it does not hide photos from the guests who have it, and it is not a password for the host: the host link and the dashboard are for that. Guessing is slowed down on purpose, so a short PIN is fine for a party.