P.H.
I've been a member since December 2009. Back then, the 4-person membership was $99, and now it's $150. They probably raised the price of that membership because of us! We've been there so many times, they're absolutely losing money on our membership! :)
If you have a 4 person membership, you can bring anyone as your 4th, and any guests (that are over 1 year old) over the 4th pay just $10 to get in. The six person membership is $30 more than the four person membership. The bigger membership is worthwhile if:
* You plan to have more than 3 guests over the course of the year
* Your guests come in packs of 2 or more
* You would have paid for your guests
To that second point, remember, you can get the fourth person in for free. So if your sibling visits three times over a year, it's not worth the extra membership. If you're going to bring in 3 friends at once, then it is.
Hope that helps! We love our membership and will definitely renew this upcoming December!