Essentially, access to a market is provided simply by a broker 'sharing' an account with you. They can manage risk at that macro level but you are free to create sub-accounts and divvy up your risk and pnl as necessary.
Obviously this does mean the broker needs to be on the platform. We are working hard to onboard as many brokers as possible and we hope encouragement from the user community will be additional incentive for them beyond the great improvements we've made in risk and administration and the elimination of infrastructure costs.