Access to see streams is controlled by security rules. If you go into Streams in QMC, select the stream of interest and click on "edit" you will see "Security rules" listed under "Associated items" on the right hand side of the page. Clicking on that will give you visibility of the existing rules determining access to the stream. You will need to either modify an existing rule or create a new one to allow access to you Stream for user2.
With respect to your other questions, one solution would be to create two new streams, "Finance" and "Sales". You would then need to create appropriate rules to determine who can access each stream and the apps within it. It is also possible to create rules for specific apps and control access that way, it is up to you what is best in your situation. There is a wizard for creating rules and lots of instructions included in the Help site.
There's also an excellent video on streams that may you may find useful located in the resource section of this site here: New to Qlik Sense Enterprise Server Videos
You probably need to create a new security rule. For example, if you stream is called "Test", in the QMC navigate to Streams and then select the "Test" stream and click the Edit button. Then as I described above you can access the related security rules. You should also see a button to "Create associated rule", if you click on that the rule wizard will appear and you can enter something like this:
In the example I've included "user2" will now have access to the stream "Test". If you wanted all users to have access to the stream you could change the "=" to "Like" and the value to *, the rules can be constructed in a very flexible fashion.