It's easy.
I will only show you the way to do it if you require email validation.
In register.php find: (might not be exact. I have hacks. Look for the general idea of it.

)
PHP Code:
// put user in moderated group
$DB_site->query("UPDATE user SET usergroupid=4 WHERE userid='$u'");
eval("standarderror(\"".gettemplate("error_moderateuser")."\");");
} else {
// activate account
Add after:
PHP Code:
$subject="Insert Subject Here";
$msg ="$userinfo[username],\n\t I just wanted to take a moment to welcome you to our board. (Replace with your message)";
$DB_site->query("update user set pmpopup='1' where userid='$u'"); // Makes ALL PMs popup for the user
$DB_site->query("INSERT INTO privatemessage (userid,touserid,fromuserid,title,message,dateline,showsignature,iconid,messageread,folderid) VALUES ($u,$u,(Insert UserID of sender here. Not Name! Number),'".addslashes(htmlspecialchars($subject))."','".addslashes($msg)."',".time().",'1','0',0,0)");
NOTE: There are three places that you need to modify for your own board. Subject, message, and user id of the sender. Do NOT use the user name. Use the member number!
Amy