Serana seems to lose her "come with me" dialogue rather early on. I was reluctant to dismiss her, if there was no way to get her to follow again. Saw a lot of console code that just didn't work. After monkeying around for two days, I found one that did work.

I used hard saves to verify it wasn't any left over console commands.

I brought her up first--
PRID 03002b74, then hit enter,
Then, typed in "setpqv dlc1npcmentalmodel lockedin_var false"
No more, and no less.
No ".", or = signs--it's just as you see (without the quotes, of course).
Hit enter, then ~ to escape the console. Opened her dialogue, and got this:

http://cloud-2.steampowered.com/ugc/937009251953936637/2FC84D84E7D0B8B8386DE166B1E6CA75F84D7356/

I turned the machine off, then back on to see if that command stayed--it did.

I had Mjoll The Lioness with me when I asked her to follow again. She said to "get rid of my little friend" first. I found her attitude rather snobbish.
Dropped off Mjoll, and picked up Serana. All she said, was "Let's go make stories", and in a flat monotone, "don't really give a crap" voice.
I was able to have a second follower while I had Serana...I haven't tried asking while Serana is with me, but I know she's not game to follow if you already have someone. I didn't realize how crabby Serana really was--I might just leave that ol' bloodsvcker back at the barn, and go get Mjoll--she never snivels about the weather or carrying stuff, and is an outstanding fighter.