Permutations occur in cases where the order of something changes the arrangement (e.g. Choosing letters of a password). Combinations occur when the order of something does not matter (e.g. choosing three donut flavors).
1. Since the order of which you choose and invite your friends does not change who you've invited, this is a combination (order does not matter).
2. When ranking a top-10 list, the order of your ranking affects the ranking itself and makes a difference (order matters). Therefore, this situation is a permutation.
This is a question of combination. In other words, the question requires determination of number of ways in which 12 appetizers can be chosen form a total of 17 appetizers.
Therefore, Possible number of 12 groups = 17C12 = 17!/[12!*(17-12)!] = 6188
