To what extent is earning from online advertising companies permissible, which pay for visiting links and viewing permissible advertisements, while giving additional sums to whoever brings in other subscribers and for purchasing a premium membership that increases profits, knowing that there is no pyramid marketing?

If the subscription is free and advertisements are permissible in Sharia, then there is no objection to earning profit from viewing advertisements, nor from attracting other subscribers and receiving a commission for that. However, purchasing a premium membership is not permissible, as it involves gambling and gharar (excessive uncertainty).