There are a few ways we use but they are a bit clunky - at an individual level an admin can add content via the access tab of a user profile

scaling up from that you can do a bulk import into the system and assign users specific content at the end of the upload steps

Unfortunately there isnt a way to select a list of users already in the platform and assign content to them in bulk. Beyond that there is the API which can process enrollments but that is done outside of the platform by someone who is comfortable using them.