Likely as a consequence of the tighter integration of location sharing in iOS 8, Apple made an interesting change to its Find My Friends app, basically tying it to the primary iCloud account configured on the iOS device’s settings, rather than allowing it to use its own, independent Apple ID. If you were using a separate Apple ID for Find My Friends from the one that you use for your other iCloud services such as Calendars and Contacts, you’ll be stuck in a situation with data in two different accounts, and you most likely had to leave your friends behind when you upgraded to iOS 8.
The good news is that while it’s not obvious, Apple has provided a way to move your friends from another account into the one currently being used by Find My Friends. You can do this by tapping on your own image where it says “Me” at the bottom of the main Find My Friends screen to bring up your settings, and then scrolling to the very bottom where it will show you which Apple ID you’re signed in with — there it should provide a link to “Move friends from a different Apple ID.” Simply tap on this and and the Find My Friends app will take you through the process of logging into another iCloud ID so that you can move your friends list from there into your current one. You can repeat this step as often as you like to consolidate your friends lists into a single account.