In the first episode of Steins;Gate 0, Okarin says something about going on group dates, with Feyris and Rukako yelling at him for doing that.
Is a group date any different from what I think it is, just a date with many people?
When asked by Feyris what Okarin's plan that day, he replied with "gōkon".
Is a group date [...] a date with many people?
If "a date" implies "those who are already in relationships", then possibly this is the source of the misunderstanding.
According to Wikipedia,
Group dating is a modern pattern for dating where a group of single men and a group of single women organise a night out, with the hope of forming romantic partnerships. It is most popular in Japan, where it is known as gōkon. In other cultures, group dating is becoming more popular as a safe alternative to single dating (especially blind dating), also helping to ease tension, because both parties will feel more comfortable having the company of their friends.
In Japan, a gōkon (合コン) is a group blind date, typically used to form at least some friendships between two groups that are each of a single sex. Generally, a single man and woman who know each other organize the gōkon in advance, each agreeing to bring three or four eligible friends. The venue is usually a restaurant, an izakaya, or anywhere people can eat, drink and make a bit of noise.
Or alternatively, from Jisho
joint party (e.g. by students from several colleges); combined party; mixer; two or more groups, esp. female and male students, combining for a party
However, the real reason why Feyris and Rukako were yelling and surprised was
Okarin's 180-degree image change. Compared to his chuunibyou/otaku personality in the original Steins;Gate, he's now living (or looks) like a normal college student (i.e. having a social life, which is something that otaku usually don't care). Just before this, they were also surprised about his choice of school club ―which is tennis club― instead of Feyris' guess of UFO/UMA research club.