Más o menos sería así:
Mete esto entre <head> y </head>:
Código HTML:
<style>
#usuarios{ display: none; position: fixed; background: #fff; margin-top: 25%; margin-left: 50%; z-index: 10;}
</style>
<script>
function mostrar(){
document.getElementById('usuarios').style.display = 'block';
}
function cerrar(){
document.getElementById('usuarios').style.display = 'none';
}
</script>
Mete esto entre <body> y </body>:
Código HTML:
<div id="usuarios">
<a href="#" onclick="cerrar()">Cerrar</a>
fjal</div>
<a href="#" onclick="mostrar()">Conectar</a>
<br />jafj
<br />jafj
<br />jafj
<br />jafj
<br />jafj
<br />jafj
<br />jafj<br />jafj<br />jafj<br />jafj<br />jafj<br />jafj<br />jafj<br />jafj<br />jafj<br />jafj<br />jafj<br />jafj<br />jafj<br />jafj
<br />jafj<br />jafj<br />jafj<br />jafj<br />jafj<br />jafj<br />jafj<br />jafj<br />jafj<br />jafj<br />jafj<br />jafj<br />jafj
<br />jafj<br />jafj<br />jafj<br />jafj<br />jafj<br />jafj<br />jafj<br />jafj<br />jafj<br />jafj<br />jafj