“Membership” means a lot of things ranging in complexity from just registering users on your site and perhaps requiring them to login, restricting content perhaps to certain levels of membership to full-blown subscriptions for premium content.
I will evaluate your requirements and give you the best and most cost-effective solution. This might simply be allowing for users to be registered on your site so that they can subscribe to a newsletter (where you can deal with the administration of payments etc outside of your site). Add-on membership systems abound, but most will allow for setting up of regular subscriptions through PayPal and Stripe etc. Depending upon your site, you may need to integrate with your merchant functionality or perhaps a learning management system for premium course material.
Most of these require some form of monthly/annual licencing which will require adjustments to your care plan to cover the extra licencing costs. The site development will also require extra work to integrate with your site’s functionality.