Running modules for different lines
Keep a timeline of what is going on with the others groups around the same time of the adventure, this way you can add flavor to the game by indicating what they might have heard, be it news, or rumors...
You can also have them be involved by having friends or contacts in said other groups, so there is more interest since he my best bud just got a promotion, or what the heck someone attacked my friend's wife..etc. It really depends on how your players play, since it can be a rich source of role play if they are of that role rather than role play.