Give them shared or mutually referential aspects!
There are a couple ways to do this, and Fate Core suggests one of them already, so I'll talk about it first.
Core's default character generation guidelines (which draw on previous Fate iterations like dresden-files) have players creating relationships between their characters as part of establishing backstory. Although this is often difficult to do and I don't always encourage it in my games, it has a wonderful side effect which I do encourage: characters come into play with aspects that connect the group together into a team.
Character aspects which directly translate to intraparty cohesion are awesome for keeping the group moving together through the story even if just one character's plotline is currently in the foreground. This kind of aspect should be crafted to create justification for tagging along, and it shouldn't be hard. Friendships, debts unpaid, mutual interests like sharing an ally or an enemy, all create bonds which lead characters to help each other. The Aeon Wave pre-made characters come with banks of relationship aspects to choose from like Anti-Megacorp Freedom Fighters and Former employees of Red Sea R&D; each player picks two, one for the player to his left and one for the player to his right.
A slightly more blunt way to do this is to give everyone on the team the same aspect, which can then be compelled as a group to get them moving in the same direction. This can be fun if you're clever about wording the aspect to mean different things for different characters. For example, the DFRPG adventure module "Night Fears" has pre-made teenage characters who all have the aspect Not a Kid Anymore.
You could also consider a "team aspect" or a setting aspect which brings the group together, but that can be trickier to advise generally rather than on a game-by-game basis. atomic-robo does a good job modelling it in the specific case where all the PCs are working for a single organisation, and adds group consequences as another shared element to promote party interaction.
At the end of the day, all these are ways to codify and mechanise solutions to the "druid problem" described toward the end of Making the Tough Decisions:
Try to never just say, "My character isn't interested in that adventure." A lot of people mistake this for good roleplaying, because you are asserting your character's personality. Wrong. [...] One of your jobs as a player is to come up with a reason why your character would be interested in a plot.
[...] Maybe, just maybe, the other PCs are your friends and you are willing to help them just because. Too often that last part is forgotten; I don't think anyone reading this has never spent the night doing something they'd rather not because a friend asked.