Admittedly, I'm much newer at this than some of you, but here's what we've done:
When we were guests at my brother's timeshare, before I had ever purchased my own, I offered to pay for the rental car. Over the course of the week we traded off who paid for meals, or we split the bill. We were two couples, and it worked out nicely. We've since traveled together, and this sharing thing works well for us.
When I invited a friend to accompany us to stay in one of our timeshares, we gave her the smaller bedroom, drove her in the rental car we already had, charged her nothing for the experience. She paid her own airfare, and paid for many of her own meals (woman loves to drink!), so it wasn't a financial burden for us. We didn't feel taken advantage of, since we invited her to come along with us. (The experience wasn't so great because of her personal habits and lack of interest in what we'd all previously planned to do, so we haven't invited her to come along since. But that's a topic for another thread.)
With other friends we've invited along, the invitation was expressed as "Hey, we've got this timeshare trip we're planning, and we'd love to have you come along. All you'd have to pay for your share is..." And the deal has been struck. Only a few have agreed, and most have said, "Sounds great, but I don't know if we can afford that right now." So we've asked others to go instead. There are no hard feelings or misunderstandings, and everyone knows what to expect. Many of our friends don't take formal vacations, so the lack of money may be a legitimate thing for them.
And if we end up going alone, well, that okay, too. We were going anyway.
Dave