We do these types of accounts. They are generally pretty simple, but you need a sound, detailed contract. Once you have that, you act in accordance with it. If you want to get into the line of business, have an attorney assist you with the documents (which really drive the procedures).
At my institution, in general, the bank receives money and waits for pre-established agreements and minimum offerings to come in (or the offering date) before doing anything. In our experience, the Private Placement Agent does the leg work in making sure investors fill out necessary forms. If the offering is met, the money is releases to the investee once proper documents are submitted. If it is not met by a certain date, the bank must return the money to the investors individually.
In your research, try also searching "Escrow Accounts"... Accounts where investors/investees act in accordance with a Private Placement Memorandum are a type of escrow account.