Sooner or later, if you're going to follow the advice of not keeping single rats, you'll end up wanting to introduce new friends to your rats. The difficult part here is that rats are such little individuals that every intro goes differently, but there are a number of tips and tricks you can use to help the process along.
Bear in mind that sometimes it just doesn't work, so if you're bringing in new rats you need to have a plan B in case you find yourself with two groups.
Combining two groups of kittens under 13 weeks or so is generally just a case of popping them into a cleaned out cage and letting them make friends.
Most adult bucks will accept young kittens under 9 or 10 weeks or so really easily using the bare cage intro (below). There's a fairly narrow window of time where the kittens are old enough and robust enough to introduced, but young enough not to smell like a challenge to the older rats. It helps to add two or more kittens at once so that they have an ally and playmate.
There is, of course, potential for the youngsters to be hurt or killed if things go wrong, so you do need to take care. Some adult bucks just don't want kittens added to their group, but mostly they will flip and smell the babies and carry on. Problems sometimes arise when the kittens don't know when to stop, and continue to bug their elders even when they've been warned off.
Some does can be reluctant to accept new kittens into the cage, and if initial intros don't go too well, just waiting until the kittens are a little older and able to stick up for themselves can make a big difference. Again, I would recommend bringing in two or more kittens so they have a playmate of a similar age.
This is generally the most difficult intro to do successfully, but sometimes you're lucky and it just works. Neutering can help to reduce the overall testosterone levels to make intros easier, but it's probably not something you'd want to do routinely.
This is more likely to succeed than adult buck intros, but again it's really down to the temperament of the rats.
This can be a useful way to let two groups get to know one another, or to let adults get used to the smell of babies. It does help to allow new rats to 'settle' into your routine and spend some time on the same diet at your previous rats.
I use this method a lot. You take a fairly small cage that neither group is used to, add substrate, food and water, and put all the rats in together. If you're worried about how it will go, have the cage in a rat safe area with the top unclipped. Have a water spray bottle handy to surprise them into stopping if a fight does break out, and a cushion to separate rats if it really goes badly.
Keep them together in this small cage until they are sleeping in one heap. This might happen almost immediately, or it may take a day or two. Although they will make you feel very guilty for confining them in a tiny cage with strangers, the stress of being in a small space will help them to start to accept one another.
Once they're getting along well, you can move them into a bigger bare cage - either the one they'll be living in eventually, or if that's really large then an intermediate sized cage. If the cage belongs to either group, clean it out thoroughly. You can then start to add hammocks, ropes and hidey holes to the cage, but if they start to fight over these then take things back a step until they calm down again.
Once you've done a few intros you will begin to know when you can speed up this process and when you need to take it slowly. Don't worry, if it goes wrong you can always back up again.
This is another popular way of introducing new rats. Find a neutral, rat safe place and allow the rats to meet one another for five minutes at a time, for a couple of times a day. Gradually extend this time until you're happy that they're getting along. Once they seem to have accepted one another, they can move into a cage together. It helps if the cage is either new to both groups, or has been cleaned out and rearranged. Don't add any defendable hidey holes until you're sure they're integrated.