I formerly worked as an executive assistant for someone at his home. He then offered me a position in his new start up company. I hired someone for my position at his home who was organizing his appts. etc.

I need to write a letter to all contacts in various organizations to introduce myself (in some cases, again), and ask them to copy me on all correspondence.

Any help would be appreciated. Thanks!

I would use email, and the various examples available in Microsoft's Office Template portal are very good as a starting point for your letter. I would re-phrase so it sounds like it is written in your voice, but you will get the idea from reading the examples in the link above.