So basically

12 members have 1M (the amount that they share together)

If the final amount they have is less than 1M, for example 12 members balance is 300000, then they will split that amount between them. The remaining 700000 will be Jeonghan's.

Next...

If the final is minus meaning they gave more than 1M to JH, (ex they give 1.5M), they'll have to pay 500000 with their own money to JH.

1.5M ( they unknowingly gave to JH) - 1M (amount of they initially have) = 500000(remaining debt to be paid to JH using their own )

If the 12 members final is positive (meaning they "steal" from JH). For ex final amount is 1700000, Jeonghan will get 1M to himself (I assume the members get nothing)

If the members write minus then it means they're giving their money to Jeonghan, if they write + meaning they're "stealing" from Jeonghan.

For example:
- if they write -200, it means they'll have 800000 left in their account
- if they write +200, it means that have 1.2M

Did I understand it correctly?

Either way, Jeonghan will get some amount of money. He has nothing to lose