Both approaches work great!
We've found that casting the widest net possible has led to the most active boxes, and sometimes unexpected contributions.
You can also:
- Ask participants to help you invite more people by looking for the "Invite Friends" button in the upper-right corder of the group page.
- Send out reminders to friends you've already invited.
If you keep the invited group small and intimate, remember to ask each friend to contribute five or more cards. Also, when you're counting on fewer people, they need to be reminded to add cards more often.