With iOS 8, the answer to this question has changed. Find My Friends is now integrated with iCloud. So if you go into Settings > iCloud, at the bottom is Share My Location, so you see that Find My Friends is now integrated instead of being its own app.
Unfortunately this also means you can no longer have a separate iCloud login for Find My Friends than you have for your iOS device's primary iCloud account.
So the answer also to the last 2 posts is, after upgrading to iOS 8, your Find My Friends will always be the Primary iCloud account you have on your device, as in Settings > iCloud.
Although you can add additional iCloud accounts in Mail, Contacts, Calendars, those secondary iCloud accounts are only able to sync Mail, Contacts, Calendars, Reminders, and Notes. Secondary accounts can NOT be your Find My iPhone, Find My Friends, PhotoStream, Backup, iCloud Drive, Safari, Passbook, Keychain.