You could use an onclick event handler in order to get the input value for the text field. Make sure you give the field an unique id attribute so you can refer to it safely through document.getElementById():
If you want to dynamically add elements, you should have a container where to place them. For instance, a <div id="container">. Create new elements by means of document.createElement(), and use appendChild() to append each of them to the container. You might be interested in outputting a meaningful name attribute (e.g. name="member"+i for each of the dynamically generated <input>s if they are to be submitted in a form.
Notice you could also create <br/> elements with document.createElement('br'). If you want to just output some text, you can use document.createTextNode() instead.
Also, if you want to clear the container every time it is about to be populated, you could use hasChildNodes() and removeChild() together.
<html>
<head>
<script type="text/javascript">
function addFields(){
// Generate a dynamic number of inputs
var number = document.getElementById("member").value;
// Get the element where the inputs will be added to
var container = document.getElementById("container");
// Remove every children it had before
while (container.hasChildNodes()) {
container.removeChild(container.lastChild);
}
for (i=0;i<number;i++){
// Append a node with a random text
container.appendChild(document.createTextNode("Member " + (i+1)));
// Create an <input> element, set its type and name attributes
var input = document.createElement("input");
input.type = "text";
input.name = "member" + i;
container.appendChild(input);
// Append a line break
container.appendChild(document.createElement("br"));
}
}
</script>
</head>
<body>
<input type="text" id="member" name="member" value="">Number of members: (max. 10)<br />
<a href="#" id="filldetails" onclick="addFields()">Fill Details</a>
<div id="container"/>
</body>
</html>See a working sample in this JSFiddle.