Try this;
<if $mybb->user['usergroup'] == "1" then>
<form action='member.php' method='post'>
<font color="FFFFFF">Usuario: </font><input type='text' name='username' size='9' maxlength='30' />
<text="FFFFFF">ContraseƱa: </text> <input type='password' name='password' size='9' />
<input type='hidden' name='action' value='do_login'>
<input type='hidden' name='url' value='index.php' />
<input type='submit' class='submit' name='submit' value='Conectar' /></form>
</if>