What a friend link does, what it lets you see, and how to tell whether you are signed in on this device.
A friend of the site sends you a link. Tapping it signs you in on that device for a year and lands you on whatever they were sharing — there is nothing to type and no account to make. The link carries the friend password inside it, so treat it the way you would a password: fine to keep, not for posting anywhere public.
If you would rather sign in by hand, the door is at traveller.org/friends/ — it asks for the friend password once.
Open an unlisted album, or a public one with friends-only photographs in it, and the purple bar at the top offers Share with a friend. On a phone it opens the share sheet straight into Messages; on a computer it copies a link. The link signs the next person in the same way yours did.
Look for the purple band at the top. No band means this device is not signed in — the link only works on the device it was tapped on, so a link opened on your phone does nothing for your laptop. Tap the link again on the device you are using, or sign in at traveller.org/friends/.
If the band is there but an album seems short, pull down to refresh: pages you had open before signing in can show their public version until they are reloaded.