Enabling users to post

Hi!!
I know a few sites which use their WP powered sites as bookmarking sites or some other type which allows the posts from anonymous or logged in subscribers . I don’t understand that how these sites like http://nestdev.com/ and http://wordtaps.com/ allow the posts from normal users .Could any one help me to do this ?

Am I need to edit functions.php for this ? Does it works if I use this code –

function submit_post()
{
	if( current_user_can( 'read' ) )
		return true;

	return false;
}

Also I have another follow-up question the websites http://nestdev.com/ and http://wordtaps.com/ have a button named “submit post” .When a logged in user click on it .A form with data fields appears on the same page as a popup to post.It is not a normal popup window .How do I make a form like that?
Thanks !