With more than one row of people it will be challenging with just one speedlight to light people properly.
P mode can be a good starting point actually unless you have a light meter. Set camera to P mode, fire off the flash exposure lock (FEL button next to the shutter button) which will lock exposure to a good point. Take a note of what the settings are, go into manual and then adjust DOF as you go. 40-50mm f8 quite often works, just adjust the ISO upwards to compensate for the smaller aperture. It is far better to bump iso one or two steps than having the back row slightly out of focus. Problem here is relying on the flash TTL which isn’t always perfect.
Check the histogram in the camera to see if it is roughly exposed properly and adjust accordingly.
The other option that I think works well if you can fire of a couple of test shots is to go into manual mode for both camera and flash. Flash at 1/2 power, f8 and iso 800. Adjust accordingly, once you’ve found the right settings stick the flash on full power. If it is more than two rows you probably need f11 or even f16 and if there are more people than you can keep an eye on make sure you take a number of photos as there will always be people looking the wrong way and or blinking.