function bp_core_activate_signup( $key ) {
	global $wpdb;

	$user = false;

	// Multisite installs have their own activation routine.
	if ( is_multisite() ) {
		$user = wpmu_activate_signup( $key );

		// If there were errors, add a message and redirect.
		if ( ! empty( $user->errors ) ) {
			return $user;
		}

		$user_id = $user['user_id'];

	} else {
		$signups = BP_Signup::get( array(
			'activation_key' => $key,
		) );

		if ( empty( $signups['signups'] ) ) {
			return new WP_Error( 'invalid_key', __( 'Invalid activation key.', 'buddypress' ) );
		}

		$signup = $signups['signups'][0];

		if ( $signup->active ) {
			if ( empty( $signup->domain ) ) {
				return new WP_Error( 'already_active', __( 'The user is already active.', 'buddypress' ), $signup );
			} else {
				return new WP_Error( 'already_active', __( 'The site is already active.', 'buddypress' ), $signup );
			}
		}

		// Password is hashed again in wp_insert_user.
		$password = wp_generate_password( 12, false );

		$user_id = username_exists( $signup->user_login );

		// Create the user. This should only be necessary if BP_SIGNUPS_SKIP_USER_CREATION is true.
		if ( ! $user_id ) {
			$user_id = wp_create_user( $signup->user_login, $password, $signup->user_email );

		// Otherwise, update the existing user's status.
		} elseif ( $key === bp_get_user_meta( $user_id, 'activation_key', true ) || $key === wp_hash( $user_id ) ) {

			// Change the user's status so they become active.
			if ( ! $wpdb->query( $wpdb->prepare( "UPDATE {$wpdb->users} SET user_status = 0 WHERE ID = %d", $user_id ) ) ) {
				return new WP_Error( 'invalid_key', __( 'Invalid activation key.', 'buddypress' ) );
			}

			bp_delete_user_meta( $user_id, 'activation_key' );

			$user_already_created = true;

		} else {
			$user_already_exists = true;
		}

		if ( ! $user_id ) {
			return new WP_Error( 'create_user', __( 'Could not create user', 'buddypress' ), $signup );
		}

		// Fetch the signup so we have the data later on.
		$signups = BP_Signup::get( array(
			'activation_key' => $key,
		) );

		$signup = isset( $signups['signups'] ) && ! empty( $signups['signups'][0] ) ? $signups['signups'][0] : false;

		// Activate the signup.
		BP_Signup::validate( $key );

		if ( isset( $user_already_exists ) ) {
			return new WP_Error( 'user_already_exists', __( 'That username is already activated.', 'buddypress' ), $signup );
		}

		// Set up data to pass to the legacy filter.
		$user = array(
			'user_id'  => $user_id,
			'password' => $signup->meta['password'],
			'meta'     => $signup->meta,
		);

		/**
		 * Maybe notify the site admin of a new user registration.
		 *
		 * @since 1.2.2
		 *
		 * @param bool $notification Whether to send the notification or not.
		 */
		if ( apply_filters( 'bp_core_send_user_registration_admin_notification', true ) ) {
			wp_new_user_notification( $user_id );
		}

		if ( isset( $user_already_created ) ) {

			/**
			 * Fires if the user has already been created.
			 *
			 * @since 1.2.2
			 *
			 * @param int    $user_id ID of the user being checked.
			 * @param string $key     Activation key.
			 * @param array  $user    Array of user data.
			 */
			do_action( 'bp_core_activated_user', $user_id, $key, $user );
			return $user_id;
		}
	}

	// Set any profile data.
	if ( bp_is_active( 'xprofile' ) ) {
		if ( ! empty( $user['meta']['profile_field_ids'] ) ) {
			$profile_field_ids = explode( ',', $user['meta']['profile_field_ids'] );

			foreach( (array) $profile_field_ids as $field_id ) {
				$current_field = isset( $user['meta']["field_{$field_id}"] ) ? $user['meta']["field_{$field_id}"] : false;

				if ( !empty( $current_field ) ) {
					xprofile_set_field_data( $field_id, $user_id, $current_field );
				}

				/*
				 * Save the visibility level.
				 *
				 * Use the field's default visibility if not present, and 'public' if a
				 * default visibility is not defined.
				 */
				$key = "field_{$field_id}_visibility";
				if ( isset( $user['meta'][ $key ] ) ) {
					$visibility_level = $user['meta'][ $key ];
				} else {
					$vfield           = xprofile_get_field( $field_id );
					$visibility_level = isset( $vfield->default_visibility ) ? $vfield->default_visibility : 'public';
				}
				xprofile_set_field_visibility_level( $field_id, $user_id, $visibility_level );
			}
		}
	}

	// Replace the password automatically generated by WordPress by the one the user chose.
	if ( ! empty( $user['meta']['password'] ) ) {
		$wpdb->query( $wpdb->prepare( "UPDATE {$wpdb->users} SET user_pass = %s WHERE ID = %d", $user['meta']['password'], $user_id ) );

		/**
		 * Make sure to clean the user's cache as we've
		 * directly edited the password without using
		 * wp_update_user().
		 *
		 * If we can't use wp_update_user() that's because
		 * we already hashed the password at the signup step.
		 */
		$uc = wp_cache_get( $user_id, 'users' );

		if ( ! empty( $uc->ID ) ) {
			clean_user_cache( $uc->ID );
		}
	}

	/**
	 * Fires at the end of the user activation process.
	 *
	 * @since 1.2.2
	 *
	 * @param int    $user_id ID of the user being checked.
	 * @param string $key     Activation key.
	 * @param array  $user    Array of user data.
	 */
	do_action( 'bp_core_activated_user', $user_id, $key, $user );

	return $user_id;
}