One of the things I really love about conferences is getting to meet and to connect with new people as well as catching up with folks that I already know.
When I was going to my first conferences, I was still quite shy in approaching people - and I do remember that it felt so nice when, as you're approaching one of the small tables in a coffee break that people stand around, someone shifts to make space for you to join. (That, back then, was an easy way to join a random group of people, because you obviously need some place to put your coffee cup to be able to wrangle your coffee break snack... so it didn't feel so much like I was sneaking up and trying to impose myself on other people.)
Since then, I've gotten less shy regarding the approach of new people - but I still remember how nice it was to be welcomed by someone shifting to tacitly invite me into their circle. So, since then, I have always tried to shift and make space when someone comes close to the table I'm at. And today, by random chance, I found out that there's a name for this: The Pac-Man Rule. The short version is: When you're part of a circle in a conference (whether around a table or not), always leave an open space in the circle for someone else to join. (But do go and read the full post behind the link, it's worth it.)