Ninja Forms + Groups. When a user registers for an account using Ninja Forms User Management (https://ninjaforms.com/extensions/user-management), assign them to a group (https://wordpress.org/plugins/groups) according to the value of a drop-down selection which matches the "slug" format equivalent of the group name.

ninja-group-signup.php

<?php
/**
 * Get all groups
 * Utility function
 * @author Antonio Blanco
 * @see https://github.com/eggemplo/Groups-Utilities/blob/master/get_groups.php
 */
function doublee_get_all_groups() {
	global $wpdb;
	$groups_table = _groups_get_tablename('group');

	return $wpdb->get_results( "SELECT * FROM $groups_table ORDER BY name" );
}


/**
 * Get group name by ID
 * Utility function
 * @param $group_id
 *
 * @return mixed
 */
function doublee_get_group_name($group_id) {
	global $wpdb;
	$groups_table = _groups_get_tablename('group');
	$results = $wpdb->get_results( "SELECT `name` FROM $groups_table WHERE `group_id` = $group_id");
	$name = $results[0]->name;

	return $name;
}


/**
 * Get group ID by slug
 * Utility function
 * Note: Groups don't actually have slugs, so this assumes that running doublee_build_title_slug on the name will yield the expected slug.
 * @param $group_slug
 *
 * @return mixed
 */
function doublee_get_group_id($group_slug) {

	$id = '';

	// Get all the groups
	$groups = doublee_get_all_groups();

	// Loop through and find the one matching the provided slug
	foreach($groups as $group) {
		$slug = doublee_build_title_slug($group->name);
		if($slug == $group_slug) {
			$id = $group->group_id;
		}
	}

	// Return the group ID
	return $id;
}


/**
 * Create processing hook to add to Ninja Forms submission action
 *
 * @tag doublee_add_new_user_to_group
 * @callback doublee_add_new_user_to_group
 */
add_action('doublee_user_form_process_hook', 'doublee_add_new_user_to_group');


/**
 * When user account is created using the Ninja Form,
 * assign the user to the deb group they selected
 *
 * @param $form_data array
 * @return void
 */
function doublee_add_new_user_to_group($form_data) {
	$form_fields   =  $form_data['fields'];
	foreach($form_fields as $field) {

		// Find the deb group field
		if( 'debutante_group' == $field['key']) {
			// The selected group, should be in slug format
			$selected_group = $field['value'];
			// Get the group ID
			$group_id = doublee_get_group_id($selected_group);
		}

		// Find the email address (username) field
		if( 'email_address' == $field['key']) {
			$email = $field['value'];
			// Get the user's ID
			$user = get_user_by('login', $email);
			$user_id = $user->ID;
		}

		// Assign user to group
		if(isset($group_id) && isset($user_id)) {
			Groups_User_Group::create( array(
				'user_id'  => $user_id,
				'group_id' => $group_id
			) );
		} else {
			error_log("Couldn't add user to group. Likely a problem with finding the desired user ID or group ID.");
		}
	}
}