In order to invite members to your organization, you must first verify the organization.
Once verified, navigate to your organization's setting's page.
Open the dropdown menu by clicking your profile picture, in the top right.
Select Settings
From Settings, select your Organization via the left navbar
From the organization settings page, you can select the Add Member button.
In the modal, search by handle, or username and click the invite user button.
You will see the pending invite in your organization settings screen and the user will have received an email and in-app notification to accept the invitation.