depending on total bond level a certain amont of ninjas will show up from the list.
as you level the bond level you will be able to invite extra ninjas from said list.
if you have medal you will be able to invite even more.

every time a ninja bond level hits a multiple of 5 (5, 10, 15, 20, etc) you will get 1 mission, not sure about this one but its what i read from what has been said already, pls correct me if i got it wrong.