Once you assemble a list of potential advisory board members, contact them one at a time. Make time to meet and get to know each one of them. Again, email or send a copy of your executive summary in advance of the meeting. Then, follow up promptly after the meeting with a letter summarizing a few major points you learned from them.
Lastly, don't rush, and don't feel the need to extend an invitation to everyone you meet. Rather, consider your meetings with potential advisory board members as exploratory research. The process can take six to nine months, or even more.