I'm not aware if there's method for doing this, but you can set up OE6 to send you a receipt from all those who've read your e:mail.

In OE click Tools > Options > Receipts and then check the box next to "Request a read receipt for all read messages"

You can also highlight the messages from all those people in the group who have read the mail by changing the colour of the mail.

Click Tools > Message Rules > Mail > then a New Mail Rule box opens.

Tick "Where the from line contains people" and then click the "contains people" link at the bottom to enter all the people in the group from your address book.

Next, tick the " highlight it with colour" box in the "select actions for your rule" section. This will give you another link at the bottom to click in order to select what colour you would like. The LIME one would stand out.

Name the rule and click OK.