The question is about the rule described in the Kitsur SA, which is based on the Magen Avraham sk 4.
A little introduction is necessary to understand this rule: The Mordechai on the 9th chapter of Berachot (quoted in Bet Yosef OC 175) reports in name of the Rif, that Achsanai and oreach cannot bless because this is not his own goodness. The Bet Yosef didn't find those words in Rif and thinks that somewhat is wrong in this quote of the Rif. So he skipped this opinion in Shulchan Aruch. The Bach says that this Rif is probably quoted from the Respona of the Rif. He understands that achsanay and oreach are equivalent, two kinds of guests.
The Magen Avraham in sk 4 gives a new interpretation of this rule. He understood that the Rif's opinion comes from the psak of the Rif regarding the blessing of Hatov VeHaMeitiv about rain.
He explains that the words achsanai and oreach are certainly not equivalent, achsanai is the owner, the host, and oreach is the guest.
Even the host cannot bless hatov vehametiv because of a necessary condition (according to the Rif) which is not fulfilled. According to the Rif, the owner of a field cannot bless hatov vehametiv for the rain, if he has no associate in this field, despite the fact that the field provides food not for him only but for a lot of persons.
Magen Avraham learns from this rule that for wine if there is a owner, the host and one or several guests, the cannot bless , because this is not a common boon (but the words of the Mordechai are הטבה שלו, not exactly congruent to this interpretation), and the guest too is not associated when he received one cup.
The Magen Avraham explains that if a jug is put on the table as it is the general case, and everybody can drink from his cup which was fulfilled from the jug (until he drink the wine is not his wine because we rule אין ברירה), and they drink together, every people around the table is associated for the jug and can bless hatov vehametiv.
In the case of the OP, the guests can drink freely up to a certain state of drunkenness, the control of the quantity can be interpreted as follows: the guest is an associate with a low percentage, as long as his part is not finished, he is an associate.