You can invite new or existing members to your organization by selecting on "member", then clicking the plus button on the top right. You can search an existing member and add them or you can invite them by putting in their email address.
To create a new member, put in their email address to invite them to set up an account. You can assign a role to this person now or assign the role after they have accepted.
You can view pending invitations by selecting "Invitations" from the home screen. You can view existing and accepted members by selecting "Members" from the home screen.