Any situation where people say 'but I could have a group of 9 and then 3 people could go into another room and say they were not part of the gathering' - that is clearly illegal. If you are one person in a house, and you invite over 8 people, and 3 are in one room and 6 in another, then no-one (i.e. a magistrate) is going to believe that no-one in the group of 6 met with the group of 3.
Here the situation is completely different. The groups will not meet, there is no intention to meet, and it is plausible that they didn't meet.
Take notevenat20's example of a house with 2 teenagers and an adult each hosting a group of 5. Could this be done? Well, possibly, if it was made very clear to everyone up front that there would be other people in the house that it was important they did not meet. If there were staggered start times, staggered end times, strict boundaries, separate toilets, and no one in fact met outside the groups.
Is someone going to believe that is what happened? Depends on circumstances. Ordinarily I would say no, because on receiving info that the house would be like that, it would be odd that some of the 15 people invited to the house would not offer to host, thus removing the need for so many people.
If on the other hand you told me the house was Chatsworth, that each group was meeting in a separate wing in order to decorate one specific room for Christmas, and had separate entrances, I would probably take a different view.
Maybe the law was drafted in order to deal with the fact that some people have very large houses? And possible very separate lives within that house?
Should you do it is obviously different again!