Considering how easy it is to find anything on the Matrix is SR4... ShadowrunSEA is, IMO, probably pretty much useless. Shadowland had script kiddies and clueless newbs wandering in all the time, and at least back then you needed to know what you were doing to Deck. These days, you buy a program and let it hack for you. So everyone and their dog (and spies from every major and minor corp, government, agency, and organization) are on ShadowSEA (and similar "open" networks).
I imagine that there are 4 types of BBS/Network in SR4...
1) Totally Private.
Jackpoint. One single person administrating the network, and the only ones that get in are by that person's invitation.
2) Refer-A-Friend
A step below Jackpoint, this board is invite open, but it more open. More than likely, you can't invite anyone until you've been on the board for a certain amount of time, or you've earned a certain reputation, or whatever. This is your main security gateway, really, so some boards will be tighter, while others a bit more open. Either way, you get a limited number of invites (1 to 3, maybe), and then those invites "regenerate" after a certain amount of time. If your invites lead to problems (Spammers, hackers, etc), then you can lose your invite privileges or get banned yourself. This method lets a board self propogate and grow without the main admin doing all invites himself. It provides some security while allowing for a broader, more diverse membership pool.
A lot of Torrent Sites these days use something like this method.
3) Openly Hidden Node
This is what I figure ShadowSEA is, like Shadowland before it. It's a hidden node, so you need to know where to look (or know how to search, at least). But, if you know where to look, you can register and login and start chatting. This provides a minor speedbump, and keeps out the most clueless users, but isn't really an impediment to anyone who wants to find it. Even if (Especially if) their goal is to spam or hack it. This method would likely require a fair amount of moderation to keep the riff raff at bay.
This resembles your average Message Board these days. You need to be able to find the board, and you need to be smart enough to set up a login. Not much of a deterrent, really.
4) Public Network
Just what it says, a publicly accessible network that anyone can join. You likely don't even need a login, you can likely post anonymously, or just type in whatever "username" you want to post as. Pretty much worthless.

Bull