Users fields on front end form
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:

&lt;select multiple name="fields[fieldHandle][]"&gt;
    &lt;option value="13"&gt;Username One&lt;/option&gt;
    &lt;option value="42"&gt;Username Two&lt;/option&gt;
    &lt;option value="144"&gt;Username Three&lt;/option&gt;
&lt;/select&gt;