Do you need to keep those messages as record in your database for future reference? Some of the following options make that possible, the first option does not.
Assuming that text box is a field on a layout, give that field global storage.
Or put a list of email addresses in the To or BCC box of the send mail dialog. This can be done a number of ways using either a script or a calculation depending on how your database tables/relationships are designed.
Or use relationships so that your found set of records, one for each recipient all link to the same record where you have entered the email message in a text field. Given the fact that over time, you will be sending many emails to many recipients, this requires three table to implement the many to many relationship required if you intend on keeping these messages on file for future reference:
Messages::__pkMessageID = message_recipient::_fkMessageID
Recipients::__pkRecipientID = message_recipient::_fkRecipientID
You'd send out your email from a layout based on Mesage_Recipient using the "Multiple Emails, found set" option after performing a find to pull up a found set of all the records linked to a specific message.
For an explanation of my notation, see: Common Forum Relationship and Field Notations Explained