The Members list and search plugin for Decidim adds an index page to search the members registered on the platform.
Adding this plugin to your Decidim based Rails app will add a new main menu entry Members which shows a searchable list of all members.
Add this line to your application's Gemfile:
gem 'decidim-members'
And then execute:
$ bundle
Fork it and create a pull request.
Same as Decidim, AGPL v3.