Membership is free and entitles you to report when you find letterboxes and also to report when you attempted to find the letterbox and found the spot but not the letterbox. This helps the planter know when there is a problem. If several people report the box as missing, then it is automatically archived so that the next people that are looking for clues in the area, don't waste their time looking for a box that's not there.
Also, as a member you can list the clues to letterboxes that YOU plant! Yes, you too can create a letterbox to share with the community. More boxes to find!