The moment
A group anniversary can be as simple as bringing the first photo you took together or asking everyone to remember one funny hand from the year.
A useful detail
Avoid turning it into an event people have to prepare for. A small toast between rounds may be the exact right scale.
Keep it human
If no one knows the date, pick the next gathering. Friendship does not need a verified founding document.
Let the person be the point.
A gift is optional; a sales link never replaces a useful gesture.