If they do not have an account yet, you can invite people to join your group via email from the "My Group" menu at the top of the main page. Select "Invite Others" and input the email address in the Recipients field. If you have a large amount of people to add to your group, or do not know everyone's email address, you can add people using a third option. From the "My Group" at the top of the main page, go the to "Invite others" tab. Select "Invite with printed cards" just under the blue menu bar. If users are brand new and have never registered, they can input this key code into their profile when they set up their new account, and will automatically be added to your group.
The easiest and best way to add members if they already have an existing account with BibleX is using their username. To do this, select "My Group" from the top of the main page, then select "Manage Members" and then "Add people."