Visit the Organization Settings and select the users tab. Select the invite button and fill in the required information to send a new user an invite.
You can select which of your active account subscriptions the new user has access to by toggling the sliders at the bottom of the page.
When done, hit send invitation.
Removing a user:
Log into Organization Settings page, select the Users tab, then click the pencil to the right of the name you wish to remove from your account. Choose to either edit their information, or remove them by selecting the trashcan icon.