This exercise is great for keeping a focused conversation, while having a large group of people. At any time, only a few people have a conversation (the fish in the fishbowl). The remaining people are listeners (the ones watching the fishbowl). The caveat is that the listeners can join the discussion at any moment.
Here is how I ran it last time:
The round table had 4 chairs. One chair was kept empty, available for whoever wanted to join the conversation. When someone joined the table, someone else on the table (typically the one either sitting longer or less involved on the current conversation) would leave the table. The other participants were standing or sitting near the table. This is the roundtable-fishbowl style.