How to shared users – 2 wordpress installs

Hello – is there a plugin that will allow users to register on one site and be automatically registered for a second site on a subdirectory?
ie. domain.com
domain.com/events

Multisite is not an option because the plugin being used in the 2nd site conflicts with the theme in site 1. But I want to keep users from having to register on both sites.

Thanks in advance for any guidance you can give.