Don't sign other peoples' device into your iCloud account.

What you want is to have each person have their own iCloud account, but then do all the iTunes store purchases under a separate account. This is obviously tricky if you already have a single iCloud account which has a lot of iTunes purchases. There is no way out of that.

If you're completely new to iOS, or are old enough to have an iTunes account that pre-dates iCloud and thus likely have a separate iCloud account, you are golden :)