Several months ago, group glitches like that happened every now and then. Groups would disappear, members would be glitched out of it, and so on, but for the most part, this has been fixed within the server. The friends' list glitch also happens, even nowadays, either deleting friends or adding random ones. I haven't seen it in this extent, however.
You said this happened several months ago, though, right?
The group issue should be resolved by now, but if it still happens, try restarting FH and coming back online. If the names are still gone in the group chat, then, you may want to consider deleting the group and recreating it.
As for your friends' list, I don't think this issue occurs too often anymore, so all you may have to do is just re-add your friends. You more than likely won't have to worry about this again.
Like I said, if this was several months ago, it should be fixed by now. If it does happen after you clean everything up with your group and friends' list, though, let us know.