How do you set a user's group on a registration form?

This answer was originally written for Craft 2. For a Craft 3 solution, see Ryan's answer above.

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I literally just worked through this problem last week. Ran into the same issue, so I reached out to P&T.

Turns out there is a "catch"... For a new user, the $_POST['groups'] value is passed as an empty array. And because of the order that it's processed, that empty array will actually overwrite what you've set manually.

After working through it with Brad, this is the solution that worked for me:

public function init() {
    parent::init();
    craft()->on('users.saveUser', function(Event $event) {
        // If new user
        if ($event->params['isNewUser']) {
            // If "groups" is in POST
            if (isset($_POST['groups'])) {
                // If not an array, it's an empty string
                if (!is_array($_POST['groups'])) {
                    // Set it to an array with the user group ID
                    $_POST['groups'] = array(2);
                } else {
                    // Merge in the user group ID with any existing ones.
                    $_POST['groups'] = array_merge($_POST['groups'], array(2));
                }
            }
        }
    });
}

Just wanted to add some additional info for anyone who comes across this thread. I also needed to check if, and handle accordingly if a user belonged to a specific group upon registration from my plugin.

Here is what that looks like:

MyPlugin.php

craft()->on('users.onSaveUser', function (Event $event) {
    if (craft()->request->isCpRequest()) {
        error_log((craft()->config->get('devMode')) ? 'User is getting created from CP.' : '');

        if (isset($_POST['groups'])) {
            if (empty($_POST['groups'])) {
                // User does not have a user group.
            } else {
                foreach ($_POST['groups'] as $group) {
                    if ($group == 1) {
                        $this->_registerAccount($event);
                    } else {
                        // Not who I'm looking for.
                    }
                }
            }
        }
    } else {
        error_log((craft()->config->get('devMode')) ? 'User is getting created from the front-end.' : '');

        $this->_registerAccount($event);
    }
});

If you're looking for a Craft 3 solution, I just got this set up on a project and hope it helps someone else out.

First you'll need to add a custom field to your form. If you want your user to choose which field, then go with a dropdown or radio button and adjust the plugin logic below. My scenario required me using a hidden field and auto-filling in the value based on a query parameter in the url.

<input type="hidden" name="myHiddenInput" value="1">

In your custom plugin's primary class, use this service at the top of your file:

use craftservicesUsers;

and then in the init() function add this event and adjust the code to your liking:

Event::on(
    Users::class,
    Users::EVENT_AFTER_ASSIGN_USER_TO_DEFAULT_GROUP,
    function(UserAssignGroupEvent $event) {

        // get your hidden input value, 1 for true, 0 for false
        $isInDifferentGroup = Craft::$app->getRequest()->post('myHiddenInput');

        if ($isInDifferentGroup) {
            $differentGroup = Craft::$app->userGroups->getGroupByHandle('differentGroupHandle');

            $userId = $event->user->id;
            $customGroupId = $differentGroup->id;

            // add more group ids to array in 2nd param to add user to multiple groups
            Craft::$app->getUsers()->assignUserToGroups($userId, [$customGroupId]);
        }
    }
);

Shout-out to Oliver Stark from fortrabbit for helping me with this.

If you disable the email confirmation on your site, you can also use this: (But Ryan's answer still better though)

Event::on(Users::class, Users::EVENT_AFTER_ACTIVATE_USER, function (UserEvent $e) {

    //Tapping into the post to get the user group's handle
    $selectedUserGroupHandle = Craft::$app->request->post('user-group');

    //Find the corresponding user group
    $userGroup = UserGroup::find()->where(['handle' => $selectedUserGroupHandle])->one();

    //We've found the user group
    if ($userGroup){

        $groupIds = [
            $userGroup->id
        ];

        $user = $e->user;

        //This is legacy code from the UsersController.php
        Craft::$app->getUsers()->assignUserToGroups($user->id, $groupIds);
    }

});

You will need a custom input field (or select or whatever) on your form to post the user group's handle. In my case, that was user-group.

<input type="hidden" name="user-group" value="yourUserGroupHandle">

p.s.:

use crafteventsUserEvent;
use craftrecordsUserGroup;
use craftservicesUsers;

Surprised not to see the Groupie plugin referenced anywhere here, which does exactly this: https://github.com/kinoli/groupie

(I am not the developer of it, just a happy user!)