It's ideal to keep within a reasonable distance of the other paddlers. Keep in mind that always being the last boat trying to keep up is not a pleasant experience. Sometimes this happens when a solo boat is with tandem boats. Try to set a pace that is inclusive.
Sometimes a boat has to stop for whatever reason and then hauls ass to catch up. The group kindly waits for them but when they catch up, everyone starts paddling again. Now everyone is all rested up except the boat that just did a marathon to catch up. Ensure everyone has a chance to rest.
Flatwater isn't without its own dangers. Some bad decisions on open water can create a situation where the whole group is in trouble if caught out in terrible conditions.
In rough, cold water, "the first thing that you'll lose is your formation, the second thing you will lose is your ability to communicate, the third thing you will lose is control of the situation, the fourth thing you will lose is your boat, and the last thing you will lose is your life." (Martin p.227)
Imagine multiple boats capsizing far from shore in massive and cold waves. Managing that situation in cold water and getting everyone and the gear back in their boats could be brutal. Weather can turn fast on large lakes, so more cautious routes closer to the shore are a great idea if it looks like the weather could change.
Keep on top of your food and drink intake more than you normally would. You may need to drink much more water than usual, even if you don't feel thirsty. Out in the bush, it will affect your mood more than normal. People can get grumpy just because they didn’t drink enough water.
We use the term "Raft up" to call all boats to come together and hold on to each other. It's especially handy if there are instructions that need to be shared, group decisions that need to be made, or guidance for an upcoming swift that doesn't need to be scouted.
If there's a long stretch of flatwater, we like to experiment with catamarans or trimarans to make things more interesting and social through those stretches. Sometimes we have floating lunches and pass food around a group of attached boats lazily floating down the river.