How to add a user to a Users field from front-end form?
You would need to do this with a plugin that provides a custom controller, which looks at the post data and modifies the entries’ Users fields accordingly.

The controller action would look something like this:

&lt;?php

$entryIds = craft()-&gt;request-&gt;getPost('entries');

if ($entryIds &amp;&amp; is_array($entryIds))
{
    // Get the current user's ID
    $currentUserId = craft()-&gt;userSession-&gt;getUser()-&gt;id;

    // Fetch the entries
    $criteria = craft()-&gt;elements-&gt;getCriteria(ElementType::Entry);
    $criteria-&gt;id = $entryIds;
    $entries = $criteria-&gt;find();

    foreach ($entries as $entry)
    {
        // Update the Users field value with the current user's ID
        $userIds = $entry-&gt;myUsersField-&gt;ids();
        $userIds[] = $currentUserId;

        $entry-&gt;setContentFromPost(array(
            'myUsersField' =&gt; $userIds;
        ));

        craft()-&gt;entries-&gt;saveEntry($entry);
    }
}