If you share a computer with others and everyone has their own iPhone or iPad, it could get rather messy. iTunes does not have good support for multiple accounts at the moment. For e.g. let’s say I own an iPhone and my brother owns an iPad, I would want to pair it with a separate iTunes account.
There are a few ways to get around such a situation. You can have two separate iTunes accounts and two separate iTunes libraries on the same computer. For Windows: Hold the SHIFT key when starting iTunes, it will then ask you which library to choose and you go from there. For OSX: Hold the Option key. If you have multiple iTunes libraries on your computer and don’t hold down the Option or Shift button when you launch iTunes, it will open the last library used. By default, all libraries are signed into whatever iTunes account was last used. To make sure that each person uses only their iTunes account with their library, always remember to sign out of your iTunes account when you’re done.
Another way which I prefer is to set up another OS user account, and start iTunes from there. This will isolate the iTunes account to the specific windows or OSX ID. This is useful especially if each user has different tastes in music or other media; users can maintain separate, personalized iTunes libraries and can customize their devices’ sync settings accordingly. Switching between accounts is also easy with just logging off rather than the complicated method of signing in and out of iTunes as above.
Last method, is to authorise multiple Apple Account in a single iTunes. This would allow common access to all music, apps, etc. Basically, everything will be shared. One account can be authorised up to 5 different computers. Sharing is caring right? Especially since this is all within a close group of friends and family.
This method allows you to share your purchased music, audio books, movies and other video material across your work, your home, your AppleTV, and so on. Problem is, you only get to authorize five computers and once you hit that limit, you’re out of luck. You can choose to deauthorise all computers from iTunes once you hit the limit.