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A person don't need to go through a similar experience in order to consider themselves as part of an in-group. The commonly used example in social science of an in-group are sport fans who align themselves with a specific team. The fans may have no personal experience of the sport or being part of that team, but they still view themselves as part of the in-group.

Personal experience can definitively help to form identity, but it can also be completely abstract and arbitrary. In many situations there are just an abstract proxy of an implied shared experience that never happened.

Left and right-wing voters also divide the in-group and out-group categories differently, which adds an other dimension to studies looking at empathy towards in-group vs out-group based on political alignment, and they will definitively differ when looking across borders and culture. The in-group of a left voter in the US may be the in-group of a right voter in Germany.