AdminCP -> Usergroup Manager ->
Usergroup You'd Like to Edit -> Edit Usergroup.
Look for Username HTML Markup. The first box is the opening statement, the second is the closing.
So if you want to use my CSS way of doing it (in my sig), for example, the opener would be: <span class="adminmarkup"> and the closer: </span>
You can use basic HTML in there as well, such as <font color="green"><b> for the opener and </font></b> for the closer, which would give you bold/green names, but CSS is a better and more cross-browser compliant way to do it. Plus it's easier to just change it in the CSS (one location) instead of editing each usergroup every time you want to change the markup.
https://vborg.vbsupport.ru/showthread.php?t=105598
See screenshot if you need further clarification on where to make the edit.