number of people invited in round 1 = 2
number of people invited in round 2 = 4
.....
total invited = 2 + 4 + ....
number of people invited in round 2 = 4
.....
total invited = 2 + 4 + ....
In the beginning, there are 2 friends you invited, so there are 2 people in the first round.
In the second round, each of your friends invites 2 people, so the total number of people is 2 friends × 2 new people = 4 people in the second round.
In the third round, each of the 4 new people from the second round invites another 2 people, so the total number of people is 4 new people × 2 new people = 8 people in the third round.
Finally, in the fourth round, each of the 8 new people from the third round invites yet another 2 people, so the total number of people is 8 new people × 2 new people = 16 people in the fourth round.
Therefore, if this process continues for four rounds, there will be a total of 2 + 4 + 8 + 16 = 30 people invited.