A limited partnership, in which you are the general partner, would leave you totally responsible for all of the losses, even those beyond the amount in the margin account. You can form a corporation. This may give you certain tax benefits. However, it will not serve to limit your personal liability, as you broker will require that you give a personal guarantee. You may wish to elect Subchapter S status, which would allow for profits and losses to be passed through to the individuals.
Most states allow for an entity called a limited liability company.
This is like a corporation, in that it provides for some immunity from personal liability for persons who did not perform acts which led to the losses. Although I have no personal experience with using this type of entity for trading, I am certain that your broker will again require your personal guarantee, since it would be the only way to assure payment by someone in the event of loss. You can trade the funds of a trust. Your personal guarantee will be required. Trading on behalf of someone else always requires explicit documentation of your rights to bind the persons or entities whose funds you are committing.