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What we normally do is make a new group. Create the trinkets collectible ONLY to that group, then you ADD the people to your group for each person you want to have the trinket. Then you email them the link to the group so they can pick up the trinkets. You can still post the trinket elsewhere and it will show, but it cannot ever (EVER) be collected outside of the group. Set the group to viewable only by group members...
This is my understanding of the most common way you can do what you want, but if I'm wrong someone will definitely say so! Lol.