“In my shul there’s a takanah that no one is allowed to tell someone else that he’s sitting in his seat.”
I disagree completly.
If someone pays for a seat (or membership) he has a right to expect his seat available if he gets there on time.
I agree it should be done nicely and I take it as a given that he would help the guest find an empty seat or one that is available that day .
Of course it is lifnei meshiras hadin to offer a guest your seat given that as MEno says guests are often uncomfortable.
but meikar hadin I think your Shul’s policy is incorrect.