If the requirement is that the entire group stay with the DM, then the operator should group people of similar interests together. If they don't, there needs to be a discussion before you enter the water, as to how the dive will proceed -- who will wait for whom, and for how long, etc. This is one of those things that is far better solved on the boat, because you can't really solve it underwater.
We had a similar situation in Cozumel. We were a group of five, who had been diving together all week, but on this particular day, we had a different guide and an extra diver. The guide told us he understood we were experienced divers and didn't need babysitting, but asked that we stay where he could see us, something to which we readily agreed. However, once we got in the water, the extra diver went with the DM and they took off like jackrabbits, leaving the five of us (three of whom were taking pictures) in the dust. To keep up with them, or even to stay where they could see us, would have required a total change in the way we had conducted our dives for a week. One of my buddies swam up with his wetnotes, saying, "**** the DM," and we all agreed, and proceeded with our dive as we wanted to do. When the guide realized we weren't coming, he waited at least long enough to acquire us visually.
In this case, we significantly outnumbered the single diver, so we had a "majority rules" feeling about the whole thing. In your case, with equal numbers, it's more difficult. So the real solution lies either in grouping divers differently beforehand, or relaxing the rules about having to stay together.
We had a similar situation in Cozumel. We were a group of five, who had been diving together all week, but on this particular day, we had a different guide and an extra diver. The guide told us he understood we were experienced divers and didn't need babysitting, but asked that we stay where he could see us, something to which we readily agreed. However, once we got in the water, the extra diver went with the DM and they took off like jackrabbits, leaving the five of us (three of whom were taking pictures) in the dust. To keep up with them, or even to stay where they could see us, would have required a total change in the way we had conducted our dives for a week. One of my buddies swam up with his wetnotes, saying, "**** the DM," and we all agreed, and proceeded with our dive as we wanted to do. When the guide realized we weren't coming, he waited at least long enough to acquire us visually.
In this case, we significantly outnumbered the single diver, so we had a "majority rules" feeling about the whole thing. In your case, with equal numbers, it's more difficult. So the real solution lies either in grouping divers differently beforehand, or relaxing the rules about having to stay together.