You can try something like this
<div id="groupmepopup" class="popup">
<h4>Fill in your information so that you can be added.</h4>
<form action="" method="POST">
<p>
<label>In-game username:</label>
<input name="username" type="text"></input>
</p>
<p>
<label>Email:</label>
<input name="email" type="text"></input>
</p>
<p>
<label>Game:</label>
<input name="game" type="text"></input>
</p>
<p></p>
<input name="mySubmit" type="submit" value="Submit"></input>
</form>
</div>
and then the css
p label {
display: block;
float: left;
width: 120px;
margin-right: 5px;
}
p input{
float:right;
}
p{
clear:both;
}
form{
width:20em;
}