You can access to the share tab from the BI Reports List screen by clicking on the share icon 
Example of Share tab:
Double click on one of the radio buttons in the "Share with" section to make this editable. You can then choose if you want the report to be available to selected users or everybody.
If you choose "selected users", the blue head icon on the right allows you to search for and select named users you want to share this report with. This is managed manually, so if any users join the business in the future, you will need to come back to the share tabs of each report they need to view.
Alternatively you can use the grey head icon to select users by profile, which means anyone added to UNTAP in the future with a certain profile will be given access automatically.
Under Permissions you can set if the users can view only or are allowed to edit the report. Under Actions you can delete a user from the list to remove their access to the report.