Open the left side menu and tap your Organization Name (Test Reseller on screenshot) to view it's settings
On this screen you can edit Organization Information, switch to another organization or navigate to Organization Members tab to edit users list.
Members tab displays all members of the current organization and invite new one's.
To invite new member press Invite New Member
Fill new members Email and name. Choose a role to see what permissions new member will have, then tap Invite to confirm