Before you can add a user into your organization, the user needs to be a member of any project within your organization.
If you don't have any projects, create a new project and double check that Workspace is set to your org. and not personal.
Once the project is created, go to members and invite the people you want to add to your organization.
Adding new users to your organization
1. Click on Organization in the top right.
2. Select the Members tab.
3. Find the person you want to add in the list, and click the three dots behind their name.
4. Select Add to organization
5. Choose which role this user should have in the organization.
if you are unsure which role the user should have, you can read more about organizational roles here.
6. The user is now a part of your organization, and any project they create will also be a part of your organization unless they specify otherwise. Depending on which role they received, they will also have access to the organizational panel.