In exchange to that money, you will be given a certificate of membership and will be taught how to recruit other people to join the group so that you can have your income too.
If there is no product or service being sold, that is surely illegal.
If the value of the product or service being sold is very small than the value that you have to pay, that might be illegal.
In other words, when you join a networking group and you are only given a certificate of membership, no doubt that you are in a wrong path.