Quote from EvOTraderV2:
That's correct. If they are marking up the clearing rates from the clearing firm, they must register as a broker-dealer. This is also true if the traders are trading their own funds through the firm and not the firms fund.
You would still need to register an LP, an investment adviser, etc which can run up to 6 digits depending on how many investors and/or traders you will have.
If you plan on marking up software or commissions or letting traders put up deposits, you must register as a broker-dealer. The legal partnership is only valid if the firm is trading investors money and traders receive a profit-split as compensation. The hedge-fund model does not allow traders to put up capital so that you can profit off their trades.
The definitions of these terms weren't as clear a few years ago but this definitely seems to be the case now. Many Chicago firms are not broker-dealers. It saves a lot of time and compliance costs.
Simply opening a shop and renting out seats is something that a real-estate lawyer would be more knowledgeable about since the regulations governing that business activity is mostly related to sub-letting.