I've already put some thought into this, but have never orderganized a party. The ideal party size is five: one is you, three are other theives which are still on your "too soon list" (i.e. "You have recently stolen from this person, and decide not to risk another attempt for the moment..."), and the fifth one is your next stealing target. Each theif then attempts to steal from the same four characters over and over in order, thus never wasting a click on a "too soon" target. If there are more than five theives in the party, the list (and sequence) of your target names is not only more difficult to remember, it also increases the chances that your next stealing target is not among the maximum of five names of other characaters which are shown to be on your block, thus costing you a refresh click.
Note that to add a name to the "too soon" list (which bumps a name OFF the list to become your next target), the theiving attempt has to be "finalized". A "finalized" steal is one in which the resulting message was NOT the following: "Your attempt failed. Perhaps you could try again with more success...". A successful steal or any other type of failure is considered "finalized" and puts the person on your "too soon" list.
All theives in the party should all agree to bring a certain number of expensive items to the party. The ideal item would be valuable, common and light. When we were talking about this privately a few weeks ago, Cinder suggested composiums. If I'm not mistaken, the value of the stolen goods are related to the amount of XP you gain towards your next character level, although the amount of "stealing %" you'll gain will remain unaffected by this.
Finally, all theives in the party have to be able to steal from each other. I have no idea what the rules are for this, and there seems to be more than one theory around, so unless someone can nail this down, just try to gather characters who all have about the same character level and stealing skill.