by Lanny (go PRD!)
The most important thing to know about rallies is that to be effective, you need to join with the correct hero in the first position. The right hero is one whose first expedition skill provides an attack boost. This is much more important than the hero's power, or anything else, this hero's skill is your primary contribution to the rally.
To make this simple, that means you should join with Jessie in most cases. You can also join with Jasser or Seo-Yoon. Joining with heros like Molly or Gina is hurting the rally. Also to reiterate, you must have one of these heros in the first position. Here are some exampels:
This is how your lineup should look when joining. You can use different heros for the second and third slots, generally your more powerful available.
This is wrong. For most players, Molly will be your strongest hero, but when joining a rally you should not join with her in the first spot.
This is also wrong. Even though Jessie is in the lineup, she's not in the first position. If she's not in the first position, she won't contribute her hero skill to the rally.
The answer here is a little complex but essentially: when a rally happens, most the bonuses to the rally come from the leader. Each joiner can contibute at most one bonus to the rally. That bonus is the first expedition skill ("first" being the top right skill when you look at the hero's skill page) of the first hero sent. This is why heros like Molly who do have a strong attack boost are not good leads for joining: Molly is strong if you're a rally leader but not her attack skill won't apply when joining rallies.
Also only four joiners (usually the first four) can contribute bonuses. This means if you join with a hero that doesn't contribue a useful skill, then you're taking up a slot someone else could use to contribute. This hurts the rally's overall performance. It's actually possible to send troops with no heros attached and this will be stronger than sending the wrong hero.