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Ship rats are highly social animals. Nearly all ship rats like the company of other rats, and should be kept in same-sex groups of 2 or more wherever possible. They spend much of their time grooming each other (often while standing on the groomee's head), and sleep curled up side by side.
Relationships are not always harmonious - I do know of one case where a sick ship rat was eaten alive by its cagemates. But these animals were from an inbred strain with a lot of psychological problems, and I've never heard of anything like it happening on any other occasion. Normal ship rats are civilized and sociable.
It is often possible to keep ship and Norway rats in the same group: see section on inter-species interactions.
Rats frequently form strong friendships with particular packmates, and can become depressed if separated from them. Wherever possible, avoid splitting up rats who are getting on well.
When adding new animals to a group, introduce them on neutral territory, e.g. in a carrying-box, and clean their cage thoroughly before they go back in. You can introduce does of all ages together. It is unlikely ever to be possible to introduce strange adult bucks together, but kitten bucks can be put with each other or, usually, with adult bucks - especially if the adults are their close relatives. In particular fathers are good with their own sons.
Does seem to have no sense of territory at all. They are tetchy and noisily quarrelsome, but no more so towards new pack members than to old friends. Bucks have dramatic kick-boxing matches while establishing dominance: but once they've got that sorted out they live more harmoniously than the does.
Bucks may spend a day or two sorting out who's boss, usually by swinging their behinds at each other and kick-boxing, but this shouldn't result in serious injury. Do keep an eye on new introductions, however, and prepare for them properly.
If bucks do fight in earnest they bite the base of each other's tail, mouse-fashion.
Ship rats scent-mark their cages and other objects with urine and with a thick black grease, which is said to be produced from scent-glands on the cheeks. Dominant bucks often urinate on items they wish to own - including you.
Does on heat attract a buck's attention by kneading his shoulders - like a cat "making bread", only faster. A friendly doe will knead you too: creeping up on you and then suddenly patting you in the ribs, which can be a bit startling. I've even had a doe carefully climb up my back in order to drum between my shoulders.
They'll happily do it to Norway bucks as well - which can cause a certain amount of alarm and despondency, since the Norway rat doesn't understand what's happening.
Unlike Norway rats ship rats do not like to sleep piled up on top of each other - possibly because not even another ship rat would want to sleep on something as bony as the average ship rat. Instead, they sleep side-by-side. They also sleep extremely heavily: in the wild, this probably ensures that they don't fidget in their sleep and fall out of their tree-top perches and nests.
Also unlike Norway rats, ship rats are not "thigmotaxic" (tending to stay in contact with solid objects, such as the walls). Instead of relying on stealth and therefore creeping around the edges of the room, as most Norway rats tend to, they rely on speed, agility and audacity, and will happily belt across the middle of the room, and your foot. The fact that their scent glands are (reportedly) on their cheeks, rather than on their flanks like the scent-glands of Norway rats, is probably connected with the fact that they aren't all the time rubbing their sides along the wall the way Norway rats do.