The sharing setting is on the left navigation in the SharePoint admin centre. Pick a setting you are comfortable with. Find the site in the list of active sites and set the sharing setting on the top toolbar to the same. Add the users to the site. There are numerous other settings that can be set but that will get you up and running. If you can figure it out my go to configuration is to restrict sharing to specific domains but due to the new admin centre not having feature parity it involves powershell at this time.