Hi cheinl - From what I gather that mission is awarded to folks who invite new users to your community (If you have user self registration enabled). You can look up in the mission in the Gamification console and find what Action is configured to trigger that mission. You can then go to Jive Advanced Gamification Module: Action List to find out what exactly will trigger that action.
This is for built in Jive 6 missions so there is no gamification console. However, I tested this in the community by inviting a non registered user to a group, once they registered I was awarded the points.