The following explanation shows how people are placed in the S2:
Each person has 7 spaces available in the first level (Level 1). Starting from the Host, the system will search in the organization which ones have less than 7 people on their first level and will select as the upline the one with the lowest ID number.
NOTE: The S1 and S2 systems are independent. There is no limit on the number of people you can invite.