Lots of good advice in this thread, and as you can see there is a lot of things to consider in a deal like this. Since your motivation is to grow your business and target institutional level investors, it is a great idea to bring on additional members, just as long as they bring something unique to the table. Without going into details about your operations financials, I am assuming this will be a cheaper option for you cash flow wise than going out and hiring employees (without giving them outright equity ownership upfront) to do the same tasks? I guess this also brings up the question of will these "investors" be involved with the non trading day to day operations of the business going forward? I can speak from the perspective of the institutional investor (i'm an analyst at a institutional fund) and say that being more than a one man shop is a required step to attract institutional money.
As far as the option part of the agreement goes, like other posters have said, it seems weird and a little unusual. I would not structure it where they get any sort of free ride or "long call", unless like Garachen said it is for a very short time frame and beneficial to both parties. For what a dissolution clause might entail, you need to work out if these guys will be passive or active investors. If they are passive, create some sort of option to buy back your ownership after a set time period. But if they are going to be actively involved, it will be pretty difficult to regain complete control, especially if they have put in a lot of sweat equity over the years.