There is a feature on SoD called an Adventure Band
This allows 6 people to split experience evenly, online or offline.
6 people adds 6% extra experience (not much, but it's an extra bonus which rocks nonetheless)
(Alt of my own)
Xach (Dylan)
Rekt (Evermore)
Prienae (Kizzy)
Everreal (confirm he is keeping this character? Evermore)
You guys can all experience at your own pace, or together, and never outlevel the rest.
This removes the "left behind" bullshit, while overall leveling faster. (as long as you all put into the experience gain together, and one of you doesn't just stop at some point)
I HIGHLY suggest you guys do this. If you guys catch up to Yamori's characters you can add him and remove me.