Martin,
If a passenger took a dislike to his or her dining companion/s, I doubt any of the ship's staff would force them to sit at the table. In fact, the Purser would try to make sure that all passengers were comfortably accommodated and would certainly be moved (if possible) to another table. I believe that Helen Candee was not happy with her table companions (taking a dislike to Alfred Nourney, I think) and moved to another table.
I'm not 100% sure how tables were assigned, but passengers who knew each other could certainly ask to be seated together - so I'm sure the Purser was relieved of some of his work by passengers requesting and therefore allocating their own groups. To avoid awkwardness, I'm sure two strangers would not be placed at a table for two. Instead, strangers would likely be placed at larger tables where there is more scope for conversation and interaction with a number of people. Furthermore, the Purser would attempt to put people of a certain nationality at the same table, so they could at least all talk the same language. This seems to have worked (to an extent) when Artagaveytia, the Penascos and other Spanish-speaking passengers were placed at the same table; however it was odd that Bowerman and Chibnall were also placed at that table! Someone in the Enquiry Room goofed! Canadians are also known to have been placed at tables in groups. Some cross channel passengers seem to have sat together, although there were some people at their table who did continue to New York.
All this is of course based on a voyage like Titanic's. When 1st Class on
Olympic was reasonably full, I'm sure it was a little more difficult to place everyone in groups they would like as fewer tables would be available to relocate passengers.
Out of 115 tables in the Dining Saloon, only 30 were either two or three-seater tables. I would say that these were more than likely kept/assigned to couples or associates traveling together or passengers wanting to dine alone. Mr and Mrs Straus were seated at a table for two and Mr Ismay had a two-seater table for himself.
Regards,
Daniel.