Sexing
Males have a slit-like opening between the two halves of the scrotum. It is possible to push out the penis. Just push downwards from above the penis. You must do this to check for and remove any stray hairs.
Who they can be housed with
It is extremely hard to introduce two adult boars without fights breaking out, possibly causing very nasty wounds. So is not a good idea. It is generally easier to bond a young boar with an adult or two young boars together.
Grease gland
The grease gland is just above where a tail would be. We've only seen an active grease gland in boars but have had reports of some sows having active grease glands too. The grease gland can be tricky to get clean but it can be done by yourself. If the grease gland is sore then see a vet.
We currently have 2 boars with active grease glands, JR and Smash. With these two or any other Guinea that comes to us with this problem we wash it with our normal pet shampoo. Rinse then dry.
People have also been known to use:
Swarfega: Used for removing grease and oil. Rub onto the grease spot then rinse and shampoo. Rinse and dry.