How to add multiple data in one form?

I need to create a simple application that has the following information: (it's a letters database)

The user create a new "Received letter". He chooses from a list of senders, just ONE name.

Then he chooses date, subject, etc.

And he chooses, from the same list of senders, the "RECIPIENTS", that could be MORE than one.

How to associate more than one recipients in just one form? If I use Multi-Select box, I cannot search for a name (have more than 20.000 names in a list).

Suggestions?