From what I've seen it's pretty common for families inviting young adults along who are friends or girlfriends/boyfriends of their children to pay for most, but not all expenses.
Considerations are: Financial circumstances of the family doing the inviting and finance circumstances of the young adult being invited.
The main thing I think is to just figure it all out ahead of time and communicate expectations about what you'd pay / what you wouldn't pay up front, so that they young person can make an appropriate decision. I also wouldn't want to invite a young person to do something that they really couldn't afford and expect them to pay.
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Some examples from my extended family:
My nephew who is 20 was just invited to spend two weeks in the UK and Paris with another family. They are paying for his air and lodging and told him he'd be responsible for his own food and
attraction tickets. They will in all likelihood cover a few meals too. // It's a nice deal for him, as he's not in the position where he could afford Europe on his own. In fact he's over there right now.
In June, we did an extended family trip to Kauai, HI with a party of 10. The one person who was not an extended family member who came was my niece's fiancé (a 21 year old working part time and still in college). My niece recently graduated from college and has a job in her field, but isn't flush in money either. Anyway grandpa covered the expenses for air/lodging/major tourist attraction tickets/most food for the 20 something grandchildren who are just starting off and/or are in college and offered to do the same for the fiancé. My nieces' fiancé said thank you so much / so generous of you. I'll take you up on that, but I'll cover my own air, as I have frequent flyer miles I can use. I think this earned him a few brownie points with grandpa too for making a contribution.
We are considering the same thing for upcoming vacations with our son who is a freshman in college (might we want to let him invite the girlfriend?). Anyway, for spring break to Puerto Vallarta we decided no -- as we didn't want to cover air, more expensive lodging than what we would get otherwise for sleeping arrangements we would be comfortable with, and some pricey tours (it's not in our budget for a fourth person), and the girlfriend and girlfriend's parents are not in an financial position to cover that.
For another trip we are taking to Rocky Mountain National Park / Estes Park where we will rent a large condo and airfare is relatively inexpensive for getting out there (like $300 pp), and we are already renting a minivan so have plenty of room, we are thinking about maybe letting him invite her if they are still dating and covering air, lodging, food, and having her just cover any souvenirs / miscellaneous snacks. We do really like her, and in this case this would fit just fine with our vacation budget, not be overly expensive for us to add another person, but would still be a bit of a stretch for the girlfriend financially if she were paying her way. And although her parents could pay for this, we're in a better position to cover this financially than they are, so would go ahead and do this.