Q. Considering that it is better to spend money on a mitzvah than to get it for free, is it better for one to have his own Shabbos seuda (even if he is sitting by himself) than to join someone's else seuda (for free)?
A. Horav Shlomo Miller's Shlit'a opinion is that it is better to join and not be sitting alone on Shabbos. However, you may want to gift wine or any other worthwhile item to your host and show thanks and appreciation. Then it will not be for free.
Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit'a