I founded a company but have brought on an industrial designer and engineer on board as co-founders. They both bring unique skill sets and will put in the sweat equity, I am the CEO and will work in more the business intelligence side of things alongside PM duties. I have some hesitation giving them a large amount of ownership, as that would dilute my own. They deserve credit for their contribution, but to have my own shares dwindle into the 30-40's percentage of ownership gives me concern. Looking to vest month to month over a period of 2 or 3 years. Thoughts are welcome..
You are thinking about it wrong.
Don't think of your organization as a pie. Think of it as a house.
When you add an extension (say a new kitchen) to your house, the value of your new kitchen now accounts for a larger *percentage* of your house, more than it did before.
But something else also happens. Your house is now worth a lot more.
I highly recommend you watch the series on raising money for a startup by the Khan Academy - http://robt.co/1u1wCsx