Only in the most dire of circumstances should you ask if you can share a table with a stranger. In other countries, it is completely common for others to take up empty seats at your table (even in a restaurant- it happened to me every day in India), often without even asking. But throughout the U.S., the sense of personal space is very strong and extends in a rather wide bubbble. In Boston, that bubble most certainly consumes any empty seats at their table, and probably any standing room near it too!
If someone is at a table by themselves and there are no other seats available, I would usually say that you are out of luck. If you do decide to be brave and interrupt a single cafe goer immersed in their work/reading/facebooking, don’t commit any of these etiquette faux pas:
• Obtain the free seat by trickery. The other day a girl asked me if I was using the free seat at my table (which was, at the time, occupied by my coat) or if I was waiting for someone. I dutifully removed my items from the seat and then she sat down. With me. At my table. Without being clear about her purpose. There isn’t even an option to say no in that situation, but you can be pretty sure the other party won’t be very happy about it.
• Be rude to the person who allowed you to sit at their table. If you have asked someone if you can sit at their table, you are giving them the right to ask you to leave or generally not to do other things that bother them. Don’t talk loudly on your cellphone, work in irritating (there’s that personal space bubble again) proximity to them, or talk back to them if they ask you to stop taking up so much space. The manager will almost positively side with whoever had the table first, particularly if that person is a frequent customer (believe me, I spoke to him when I had my squatter incident the other day).
• Take up excessive (or really any) space at the table. If someone has agreed to let you use their extra chair, don’t abuse their politeness by getting in their way or practically pushing their things off the table with your laptop, coffee, scone, and four textbooks.
• Chat up the person. If the person you have sat with is by him or herself, they are probably doing something, and you should not abuse their hospitality by badgering them with pointless small talk or get to know you questions. If they decide to talk to you…well, that is the price you pay for sitting at their table.
Most importantly, thank the person when you leave. I offered a seat at my table to someone else who had been waiting along time when I was lucky enough to snag a table and he was polite throughout and very thankful when he left. It *almost* made me thinking sharing a table with a stranger isn’t so bad after all.