We have an introduction/safety/warmup speech for all groups at the start of our attraction. I use a stopwatch to track time between groups, and will vamp on the speech if I need to stretch out the time. Depending on the group, I'll sometimes add in extra time between groups if it looks like it will be needed. Examples would be groups that have someone who can't walk very quickly, or a group of screaming high school girls that will take forever to get through the areas. Standard timing is about 3.5 minutes or longer on slow nights, 3 minutes on busy nights. Any faster and groups are running on top of each other. Much slower and it's really hard to keep the intensity up.
After a night or two, the stopwatch isn't really necessary, it becomes automatic.
On some individual scenes they'll have someone man the door to pace groups into the scene. Typically that's only used if some major resetting is needed.