Young Guineas (6+wks old) usually go thru a gangly teenage stage for several weeks, so it's a good idea to make sure their main diet consists of no less than 20% protein during this age range, free choice.
Cracked corn, scratch, even sweet feed will all help fatten up a Guinea. You don't want them fat tho, it's unhealthy. You want them to build muscle/meat... and protein helps with that, not corn/starches/fat.
Feeding a layer feed with 16% protein free choice is all adult Guineas really need, especially if they are free ranging also.
If they are thin there may be a worm infestation causing them to be underweight. Maybe take a sample to a vet and have them do a fecal to see if worms are the culprit.