At the top-right of your page, you’ll see a “Share” option. This lets you search for users and grant them permissions on the site:

If you select Full Control, the user will become a Site Owner and have full permissions to change all content, and control who can access and edit the site.

If you select Edit, the user will become a Site Member, who can add, edit and delete content. Your content editors should go in here.

If you select Read, the user will become a Site Visitor, with read-only permission. Add users who you want to be able to view the site in here. When you’re ready to share the site with everybody in the organisation, search for “Everyone except external users” and add them to this group.