The letting one or two birds out at a time thing never worked for me, it just mostly stressed out the loose bird or birds and created a lot of noise from the birds still locked up. And yah, what a hassle to try and get a couple birds back in while keep the others from getting out, especially once a few of them have gotten to go out a few times, they all want out each time the gate or door is open. An attached covered run is priceless when it comes to this part of training your Guineas to come in at night, I've successfully managed to train my flocks with at attached run, and without tho... using food as their motivation.
What works for me is take their feed away around noon on the day I plan to let them out, and I wait til around 4PM and open the door/gate. (Prior to this tho I have repetitively used the same call over and over every time I refill feeders or give them treats in a metal pan, so they know what the call and the feeder and feed pa sounds mean... FOOD!!!! I just sit back and let them come out on their own, I don't force them out (they may not come out the first day, but don't force them, just try again the next day). But once all the scaredy cat rubberneckers have wandered out, I calmly go close the door/gate. Then I just hang out with them. When it's time to come in (right before dark) I go in the coop, shake the feeders, call them and let them hear me pouring treats into their metal feed/treat pans. They come running. It helps to have a calm helper herd them in while you are inside calling them in. I repeat this routine night after night, gradually letting them out a little earlier and earlier each time (and calling them in at the same time each night) until they are reliably coming in when called. Guineas like routine, so if you help them establish one they tend to stick with it. And let me stress that
you really want to be sure to establish a routine of getting them back in before
they make their own routine of going up into the trees instead. Eventually you may be able to work up to just opening the gate/door for them in the morning and then just closing it at night because they will be in there and up on the roosts all on their own... I can do this, but my birds expect treats every night at lock up/feeding time before they fly up to their roosts
(they yell at me if I just shut the door/gate and don't bring yummies for them!).