You don't have to build up your profile to send friend requests to people you "know", whether it's real-life know or people from this forum or other online places. You just have to have a profile. But as Bluebird said, it's a really, really good idea to message people first and tell them who you are and where you know them from, especially if, as in my case, your Fet name isn't the same as your name on this forum. Another member here recently suggested I join a group she belongs to on Fet, and told me to check her profile to find the link to the group, but even then I messaged her on Fet and asked if I could friend her on there rather than just sending a friend request.
There's nothing wrong with sending a quick message saying 'Hi, it's drgnfly from Polyamory.com, is it cool if I friend you here?", but if you just send an unexpected friend request, it might get ignored. Also, though I'm not sure that it's true for anyone on this forum, some people on Fet have requirements that any friend requests and/or messages go through their Dom, owner, partner, whatever, and it's a good idea to make sure of that kind of protocol.
That said, you (and anyone else reading this) is welcome to friend me on Fet: KCWinter. Just message me along with the request so I know who you are.