Whenever I exit the city during this quest my followers, don't follow.

Is this normal? I have Derkeethus with me but I annoyingly started this quest in between finishing the Barbas quest.

If I lose my companions, can I get Barbas back? (I know I probably can, Derkeethus)

Ive had similar problems with that quest especially if im on the Legions side. If i recall one time Lydia dissapeared and couldnt find her.

I've got them back! Cowards just didn't want to face an army of Stormcloaks!

heh heh probably, even though im not one of them the Stormcloaks are my friends, if i see them in a fight with the Legion or someone i always come to there aid. think ive only sided with the legion a few times. Battle for Whiterun and Solitude are my favorite civil war quests.

One time I had Lydia as a follower. She was living at the time at dragons reach. I had her follow me and she did, until I went outside the gate. Don't know why

I don't know about the Stormcloack side, but if you're with the Empire, your follower(s) will stay inside the city walls once the attack begins and remain there until the battle is over. I don't know if that's a glitch or an intended feature, but you can work around it by telling them to wait just before entering the city to inform Balgruuf that war is coming. That way they'll be there for you when you run back outside to start the battle.

It's my first time doing this questline, and I loved Battle for Whiterun. I genuinely cared & wanted to defend it against the Stormcloaks.

Aah, thanks for the tip, managed to do it with a little help from a Dremora Lord instead!

Yeah, that was the point followers seem to stop following. As Kentucky says, not sure if it's a bug, seems a little odd so I think it might be.