Silk has been replaced by Bliss-Out
Many people are going back to Bliss-out and the management there has become more foreigner friendly due to pressure. There are more foreigners there now, like it used to be years ago. So reluctantly, I will recommend foreigners go there, because they don't have a speak Japanese requirement.
You still, however, have to watch out for "girlfriend stealing" by other customers (single women and men) and the staff (by getting her contact info). Make sure your girlfriend knows NOT to give out her real name and NOT to give out her contact information to anybody, including the staff. If they want to give her a business card, you (the guy and member bringing the guest) should take it for her.
If you want straight out swapping and no worries about "girlfriend stealing attempts" then Olive-21 is the best, BUT your woman should be clearly prepared and understand what's going on. Olive-21 is "hardcore", while Bliss-Out is better for beginners or newbies only wanting to "get the toe wet first".
Also, many women that date foreign men, don't like to play with only Japanese guys. Olive-21 can only have Japanese guys, while Bliss-Out will at times have foreign men. Some swinger couples prefer foreign men or a more international combination to choose from.
You might consider inviting other swinger couples that you know or swinger couples from AFF or Craigslist, to meet you at the HB or CK to ensure you have fun on the day you are going and plenty of people are there to choose from.
Bliss-out
http://www.bliss-out.jp/
Olive-21
http://www.olive21.net/top.php
Happening Bars, Tokyo (English List)
http://www.mabe-navi.com/english.htm