I understand,
Find the following code in your template postbit legacy:
Code:
<td class="alt2" width="175" style="border: $stylevar[cellspacing]px solid $stylevar[tborder_bgcolor]; border-top: 0px; border-bottom: 0px">
Replace with something similar to this:
Code:
<if condition="is_member_of($post, #)">
<td class="postbit" id="td_post_$post[postid]">
<else />
<td class="alt2"></if>
The red "#" is the usergroup.