To add users, you must make sure that you have the permissions to add new members (you must be either an owner or a Moderator).
Add user to a channel or discussion
Go to the "Members" tab in the upper right hand corner of your channel/discussion (it will look like a group of people.) Then look to the bottom of the menu that appears on the right side of your screen.
Click on "Add Users", you can search and add anyone within your school's Nectir workspace.
Use slash commands within the message bar to add a user by username, or add everyone from a specific room (aka channel)
Go to the message bar within your desired channel or discussion.
Type in /invite-all-from #your-room-name-goes-here
Once you enter this command from your message bar, all of the users within that channel will be added to your channel/discussion. This makes it easy to add all of your students from the main channel into your discussion within it.
You can repeat this command multiple times whenever you need to make sure everyone has been added.
Remember, discussions do not automatically include everyone from the parent channel.
How to see who is in your channel or check if they've been added:
Click the "Members" tab in the top right corner.
Toggle the list from "Online" to "All Users". If you have a large group, you may need to scroll down and select "Show More" to see and search the full list.