We've taken family and friends a couple of times. A few years back we had two single female friends come with us for the first half of the trip (5 days). We paid the way of the one entirely, as for the next five days she flew home with our DS and stayed at our home and cared for him while we vacationed in WDW without him <beg> The other woman I had traveled with several times before, and knew she is low maintainence and always offers to pay her way plus some. They knew that they needed to clean up after themselves, didn't get daily housekeeping so hang up those towels (with towels for four between the two over five days they didn't need laundry done), and that they would be sharing the lock-off. I told them to flip a coin over who got the bed v. who got the sofa--they settled it between themselves.
When they left, my in-laws and another single woman friend came in. I had made it more than clear from the very onset that the IL would have the lockoff, which they were quite fine with, they sleep in seperate beds at home anyways. Also told friend that she would be in LR on sleeper sofa, and she was fine with that.
Really the only problem we had was that MIL got on our nerves, like she often does--we solved that by going our own ways during the day and meeting for dinner. Everyone payed their own way, IL's picked up dinner one night and friend bought us lunch one day and had brought some items from her home area that I had wanted and wouldn't let me reimburse for them.
Last November we went with my Sister, BIL, and their two kids. Didn't stay at
DVC, but had such a good time that we've invited them to be our guests at DVC in April '03. I paid for car service from MCO, they bought the groceries. They paid for rooms at SoG, we paid for meals. Worked it out when we got home and it was even

They insited on paying us for things anyhow, we refused. They bought us Disney Dollars and told us to have dinner on them at Le Cellier as a thanks for planning the entire trip. I spent the DD on presents for their kids at TDS
At any rate, one thing I've found to be key is that if you are planning a trip with another family, make sure you are in the same socio-economic bracket and have similar spending habits. In the case of my sister's family and my family, we make the same amount of money give or take, and have the same expectations for vacation spending. We had discussed daily dining costs (this is the biggest variable!) and determined that we not only had the same budget, but also the same food tastes. We also determined from the beginning that we did not want to go the fast food route, as it was too hard with two little ones. Much nicer to let them bring it to us
We have talked to my brother about coming with his wife and kids, but have backed off on it. I very quickly realized that our incomes have a pretty big disproportion, and our spending habits are very, very different.
I haven't had the problem of someone I invited inviting another person yet, and I'm not sure how I would handle it. I know I wouldn't be pleased though. The sad part is I wonder how many times it's not really them that are at fault, that the "tag-along" had actually invited themselves, and your guest didn't have the heart or nerve to say no. Another reason we haven't invited my brother is that his in-laws are notorious for inviting themselves along. If I did invite my brother and his family, and they called and asked about the in-laws, I'd simply say no, there isn't room, but I'd be hapy to help them find a discounted room that they can stay in (and pay for). Somethig along those lines. I don't think you can be taken advantage of without allowing it.
Anne