Highlight comments made by registered users of the site

Comments that are left by registered users are given the class “byuser” – so you can create a style in your theme’s stylesheet to point out these comments. Something like:

.commentlist li.byuser {
        background:#FF0000;
}

That would make the background of users’ comments red (insert a more appropriate color).

If you are not using a custom theme, I’d recommend creating a child theme to do this.