Have all gliders been vet checked and had fecals done? Have you done the 30 day quarantine period before allowing the new gliders to meet your current gliders?
If all the above has been done, start by moving their cages side by side, but not so close they can reach each other. After a week or so, start switching sleeping pouches and toys. After another week or so, switch cages - meaning put each pair of gliders in the other pairs cage.
Then start introducing them one at a time to each other in a neutral place. The bathtub is a good place for introductions as you can watch them and separate if necessary. Dont have a sleeping pouch as you dont want them to start fighting in the pouch.