This is not a direct answer to your question, but I have always found that editing a contact group in Outlook is a big pain and not worth the trouble, especially if the list is fairly big and people are frequently needing to be added or deleted to the group. If you use Excel, even at just a basic level, it is much simpler. You can maintain the list of these addresses in Excel, easily adding or deleting members as needed.

To address an email message, In the Excel file were you have listed the addresses in a column, simply select the list by starting on the top address and pressing Shift down arrow until you have selected all the addresses. If the list is long, from the top, instead press Control Shift down arrow and that will select all the way to the bottom of the list.

Now press Control C to copy.

Open Outlook and press Control N to start preparing a message. Then press Control V in either the To or BCC field to paste them all in. Then complete the rest of the message and send it off.

David