According to this answer R' Moshe Feinstein Paskened that, since for many years the practice of Bentching on a Kos (reciting grace after meals over a cup of wine, as formally constructed in the Talmud) was abandoned because wine was scarce and expensive, nowadays we do not need to Bentch on a Kos, even when wine is readily available.
What if there is actually wine on the table, the bottle is open, and the host asked moments before it was decided that everyone would Bentch, "Does anyone want some more wine?" (In other words, one need not be shy in taking some, as the host wants the open bottle to be used and not wasted.) Is the Halachah actually that we no longer require it at all, or simply that if it's going to be a burden at all, even if it's just to go to the kitchen to get a bottle, it's not necessary to go to that trouble? Hence, if it is right in front of you, open, and there is leftover wine that's probably going to be wasted if it isn't consumed, do you have a responsibility to take advantage of it and Bentch on a Kos, or if nobody is really in a mood to drink the wine do we say that then it still isn't necessary?