Is it possible to include a Users Fields Type on a front end form? I'd like to allow people to choose existing users on a front end form rather than giving them access to the admin area of the Craft site.
Sure is! You'll want to bookmark this ridiculously handy page, but if you look under "Relations" you'll see the syntax you're looking for:
<select multiple name="fields[fieldHandle]"> <option value="13">Username One</option> <option value="42">Username Two</option> <option value="144">Username Three</option> </select>