As I understand it, jiu-jitsu (among other things, I'm sure) tends to emphasize grappling with a single attacker. Aikido tends (among other things) to emphasize movement among multiple attackers.
There are many different styles of Jujitsu. Brazilian Jujitsu is what exposed grappling to the mainstream through the UFC, but there is a lot more to jujitsu than ground work.
I studied Miyama Ryu Jujitsu, and we had many defenses against multiple attackers. The attackers would wield knives and clubs most of the time. In our school, they didn't emphasize grappling that much at all, and it was said that thats the last place you want to end up. Multiple attacks are common in most martial arts and not special to aikido.
Correct me if I'm wrong but the multiple attacks that I have seen in aikido seem more to be about movement drills and not real attacks per se.