You can use this...
<form name='member_add' onsubmit='showUser(this.member_list.value, this.category_list.value, this.description.value);'>
<input type='text' name='member_list'>
<input type='text' name='category_list'>
<input type='text' name='description'>
</form>
or if it doesn't work, use this..
<form name='member_add' onsubmit='showUser(document.forms["member_add"].member_list.value, document.forms["member_add"].category_list.value, document.forms["member_add"].description.value);'>
<input type='text' name='member_list'>
<input type='text' name='category_list'>
<input type='text' name='description'>
</form>