They can either install QS Desktop and create the app there. Or they can login to the Hub. The hub would be the address for the QS server with the /hub at the end. They should then be prompted to login.
Once there they have their own work area and can click on create app and start through the process.
Since you are using the Enterprise version of Qlik Sense your users would connect to the HUB using a browser. However, before doing this you need to have either setup a User Directory Connector to bring your Domain/Server users into Qlik Sense. This will allow you to allocated tokens to users. Once they have been allocated a Token, have them go to the HUB via a browser and it should ask then for their network credentials and access to the HUB.
Please goto help.qlik.com and under Qlik Sense you will see instructions on the above steps in more detail.
As long as they have the data connection pointing to your data then they can use that to create their app. If it was created in Enterprise then all users should have access to the connection unless you limited it by a security rule.
If they create in Desktop then they must create the connection to your data and it must be accessible from their computer to do so.