The usual tactic is that one person pays speedy boarding and saves seats. This doesn't bother me too much if they're normal seats and there are plenty of other rows available. What does bother me is people doing that for exit rows or the front few rows, as there is limited availability so those definitely should be first come first served.

It's never happened when I wanted to sit there but I have seen it. If it happened to me, I think I would just ignore them and sit down anyway (if they were in the middle, which is the usual trick), or insist they moved to let me in (if in the aisle), calling the cabin crew if necessary.

Neil