Inviting Users to Organization
First, you'll have to go to your Organization Details Page. To get there, just click on the Organization name from anywhere within that organization, and choose "Edit". You can find that near the top of the page, next to the sidebar.
If you don't see any organization name here, you may still be in your overall dashboard. Navigate to your Organization first, by clicking on it on-screen.
Once here, you'll be able to add users to the Admins field. If they are an existing user in any of your other projects or organizations, then their name should show up in a drop-down once you start typing.
When you're done, make sure you click "Save"
Any users added here will have full access to all projects, permissions, and other areas of your organization. If you want to limit their access, consider inviting them to a single project instead. For more info on that, see