The best open source alternative to Ultimate Member is BuddyPress. If that doesn't suit you, our users have ranked more than 10 alternatives to Ultimate Member and six of them is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to Ultimate Member are Paid Memberships Pro (Freemium, Open Source), WP-Members (Free, Open Source), WP User Frontend (Freemium, Open Source) and UsersWP (Freemium, Open Source).
Social networking in a box. Build a social network for your company, school, sports team or niche community all based on the power and flexibility of WordPress. Install BuddyPress directly through your WordPress dashboard plugins page, or download it manually from the WordPress.
Paid Memberships Pro offers a complete membership solution for WordPress featuring unlimited levels, recurring payments, registration & profile fields, protected member content, robust member management and more.
The Ultimate Frontend Solution for WordPress to let your users manage everything from Frontend! Effortlessly create Registration Forms, Profiles, WooCommerce Products, Subscriptions & Contact Forms with Custom Post Types support.