I had a similar issue, after digging through the code, the code in the default template is as follows:

<div class="GroupViewTop"> 
    <div class="dgdAvatar dnnLeft"><img src="[groupitem:PhotoURL]" alt="[groupitem:GroupName]" /></div> 
    <div class="GroupViewHeader"> 
        <h1>[groupitem:GroupName]</h1> 
        <p>[groupitem:GroupDescription]</p> 
    </div> 
<div class="dnnRight dnnClear"> 
[IsOwner] 
[GROUPEDITBUTTON]  
[/IsOwner] 
[IsMember] 
[IsNotOwner] 
 <a href="#" id="LeaveGroup-[groupitem:RoleId]">{resx:LeaveGroup}</a> 
[/IsNotOwner] 
[/IsMember] 
[IsNotMember] 
 <a href="#" id="JoinGroup-[groupitem:RoleId]" groupid="[groupitem:RoleId]" groupViewTabId ="[GroupViewTabId]">{resx:JoinGroup}</a> 
[/IsNotMember] 
[IsPendingMember] 
 <i>Membership Pending</i> 
[/IsPendingMember] 
</div> 
</div>

I guess the default template changed in DNN7, here is the new one.

<div class="dgdGroupQuickInfoWrap dnnClear">
   <div class="dgdAvatar dnnLeft"><a href="[groupviewurl]"><img src="[groupitem:PhotoURL]" alt="[groupitem:GroupName]" /></a></div>
       <div class="dgdGroupQuickInfo dnnLeft">
           <h3><a href="[groupviewurl]" title="[groupitem:GroupName]">[groupitem:GroupName]</a></h3>
           <p>[groupitem:GroupDescription]</p>
            <ul>
               <li class="posts-icn">[groupitem:stat_status] {resx:posts}</li>
               <li class="member-icn">[groupitem:UserCount] {resx:members}</li>
               <li class="photo-icn">[groupitem:stat_photo] {resx:photos}</li>
               <li class="docs-icn">[groupitem:stat_file] {resx:documents}</li>
[IsNotMember]
               <li class="join-group-icn"><a href="#" title="" class="dnnTertiaryAction" id="groupJoin-[groupitem:RoleId]" groupid="[groupitem:RoleId]" groupViewTabId ="[GroupViewTabId]">{resx:Join}</a></li>
[/IsNotMember]
[IsPendingMember]
             <li class="join-group-icn"><a href="#" title="" class="dnnDisabled">{resx:Pending}</a></li>[/IsPendingMember]
           </ul>
    </div>
</div>