At the time of writing, there are hundreds of plugins tagged with users in the WordPress plugin directory (https://wordpress.org/plugins/tags/users). We can actually divide those plugins into many groups as the functionality offered is exceptionally wide. For example, there are plugins for the following:
- Dealing with various author accounts, co-authoring posts, and multi-author sites
- Constructing membership sites around WordPress where members can get access to premium packages of content based on their subscription model
- Building a classic e-commerce store where everybody can make a purchase from the available directory of "things"
- Building an online forum based on WordPress
- Building an email newsletter sent to a given site's users directly from within WordPress, instead of using external services
- Launching a social network on WordPress...
