On a Shabbos a few years ago, I was davening out of my neighborhood. As we walked into the Shul on Friday night, my host told me that we can sit anywhere, except in the aisle seats, because the more involved members of the Shul sat in those seats.
In the shul where I daven, we have Makom Kevuahs (reserved seating for Davening) and when people requested seats when we first move in, the aisle seats where the most requested and generally we allocated them to the more involved members. In both cases, involved members where the ones who volunteered most in the running of the Shul, or were members for the longest amount of time or were very generous in supporting the Shul.
I don’t think in either Shul, any guest would be asked to move if they took someone’s seat, whether they were involved members or not, but I think it makes sense that when you’re a guest in a Shul, not to take an aisle set unless you know the person who normally sits there won’t be there. It’s not a hard and fast rule, but it’s a rule that makes sense if you’re like most people and don’t want to unnecessarily upset a person to any degree by taking their seat.
What seat should you take? Well the best section in our shul is the one furthest from the door. As far as which seat, it’s generally a good idea to ask any member already in the Shul, informing them that you don’t want to take another person’s seat.
I think some people will respond to this idea that any Shul member should be accommodating to guests and not care that somebody is taking their seat. While that is true, I think there is also a case to be made that the guest should try to avoid taking a member’s seat if they can avoid it.
In summary, as a guest try to avoid sitting in someone’s seat and as a member if somebody does sit in your seat, don’t make them feel uncomfortable.