To send email to multiple users, stored in multi-user people picker field, using sharepoint designer 2007, one needs to play a little trick. Please do as follows:
1) Create a variable , create a new action of type Build Dynamic string:
Click on “Actions” and choose “Build Dynamic string”. Click on “Dynamic string”, this will open a new multiline textbox. Click on Add lookup , choose Source as Current Item and select your multiple user people picker column. You will see multiple values column only while creating dynamic string. Store it in a new variable something named like “EmailUsers”.
2) Create another action to send email and in TO section , choose “EmailUsers” variable under Workflow data.
Thats it !!