Share nodes and users across multiple sites
Sharing users can be done by setting up OpenAuth or something like it. 

Sharing nodes can be done using views database connector this tool will let you pull data from a table in another database. To make it work for pulling in nodes, create a MYSQL view (https://dev.mysql.com/doc/refman/5.7/en/create-view.html) that holds the node data you need to share out.  The views connector will be able to read in that table allowing you to create views from your other server without having to duplicate the content.  You can filter on any field in the table. This set up works well for a 1 to Many Content server environment where you don't want to have a new copy of every node on every new install. 
 https://www.drupal.org/project/views_database_connector

You can also create a module that pulls data from the other database and present it any way you want. 

If you are fine duplicating the content you may want to explore Drupal aggregation it's easy to set up and can all be done through the UI.