(thanks for the invitation to comment, Naz..)
It's a bit difficult for me to speak openly, on this public board, about non-registered firms. What I'll try to do is shed some light on generic examples from current times and the recent past.
Some non-registered (non Broker Dealers) firms over the years have pooled traders money together to develop a large enough pool to keep open positions under the retail trading limits (PDT of 4 to one for example). This practice has been shown to be illegal in most States, and is a cause for concern.
Various ways of trying to circumvent the in place PDT and other margin requirements have proven to be very costly to traders over the years. Other matters come to mind, such as aggregation rules, etc. Caution should be exercised with these firms, IMO.
The question about putting down a "depsoit" can best be addressed in the way that the broker dealers do it (not just Bright). The capital belongs to the trader...the trader becomes a member of the Primary Firm's LLC (vs. a "sub lllc" or other entity). This is basically the traders "account" - to add money via profits, or to deduct money with any losses...just as with any account anywhere...IB, Schwab, etc. The Primary Broker dealer has capital from the "owners" - the Bright's in our case, shareholders etc. in the case of some other firms. This capital is subject to being considered adequate to meet "net capital requirements" for all the traders within the Firm.
The allocations of profit and loss go to each trader, the overall capital usage is dependent upon the financial viability of the General Partner (owner). This capital usage varies greatly from firm to firm.
Disclaimer: I, of course, cannot address the "legality" of any trading arrangement. I am presenting my understanding of how things work within our industry. These opinions in no way reflect legal advice, and may not reflect the opinions of Bright Trading or any of it's affiliates or subsidiaries.
Don