The Shulchan Aruch states that it's allowed to drink water on motza'ei Shabbos before havdalah.
The Magen Avroham writes that to count a tenth for zimun it is ok for him to drink a cup of water, since water is part of drinking, and drinking is part of eating.
What is the din on motza'ei Shabbos: can a person drink a cup of water to make a zimun of ten?
Answer:
As you note, the Shulchan Aruch (299:1) rules that one can drink water on motza'ei Shabbos before havdalah.
According to the Magen Avraham (197:6), who rules that water alone is sufficient to participate in a zimun (see Mishnah Berurah 12), it appears that drinking on motza'ei Shabbos will therefore be sufficient to complement a zimun.
Although the group is eating a "Shabbos meal," and drinking the water is certainly not a "Shabbos meal," it does not appear that this will prevent a zimun.
For instance, if on Friday afternoon one person has made kiddush and is eating a Shabbos meal, and another who has not accepted Shabbos wishes to join, it appears that there is no problem with eating something and joining.
Best wishes.
