Hot tip straight from the devs, through me, to you. (I was chatting with a couple of them a night or two ago, very cool guys.)
A quick way to level up a dude is to use a level 10 dude to collect as many geotags in an event as possible. The amount of credits you get from them scale to your level so your level 10 dude gets more credits than a lower level dude.
Then what you do is, don't finish the event. Just quit out. Geotag earnings are cashed in at the end of an event but the game remembers what you picked up anyway.
Now go finish an event with your lower level dude. All those geotags you got as a level 10 dude will cash in for your lower level dude. Since credits in this game = XP, you should see a big jump.
Did it last night with my level 10 Elise and my level 6 Tane; he jumped up an entire level after doing the above mentioned procedure, just one time.
Also not quite as good as that but still pretty good: you can buy level 10 geotags with a level 10 dude, but then equip them and place them as your lower level dude for lots of credits right up front.