Quote from mgookin:
Multiple traders in the same account seems to be inviting of problems, undue risk, etc. One mistake could take down the whole thing. It costs nothing to open additional accounts.
Let's say your capital is $1MM.
You could have what you may call your "parent account" where you stash the cash. Then you can set up "child accounts" where you could allocate smaller amounts of capital (starting with $25k or $50k per trader for example). This may be a safer setup. It's easy to move funds from one account to another with the same broker.
As to what brokers allow multiple logins concurrently (so they can trade and you can monitor activity/ balances) you're going to have to get on the phone/ e-mail and contact some brokers. Every broker has different systems. Some brokers e-mail out daily financial statements which may accomplish the objective. Do you intend to watch every trade/ tick? Or would daily accounting coming from the broker suffice?