A player character can theoretically rank to 51 without ever interacting with another player character. Group ranking bonuses is a functional means of discouraging that. However, if the goal of that design is to intertwine the reward cycles of ranking and PC interaction, then there may be a better way (or a secondary incentive, since they aren't mutually exclusive).

XP gains from soloable activities could be recorded but not transacted until the earner has targeted another PC in some significant fashion for good or ill. This would probably result in a few things:

1) The resources/rewards a player uses to rank (hp, mana, moves, skills, gear level, etc.) would be absolutely bounded by the level at which they last interacted with another player. This means no more major level gaps in a character's RP history.

2) The risk of losing reserved XP before or because of the next sealing interaction would encourage a strategy of continual significant interaction. As a result, individual interactions may mean more, both economically and due to relationship building. A positive feedback loop should arise.

3) This naturally encourages group ranking, since buffing or betraying an ally could quickly seal the XP earned in the group. However, solo rankers could also get their interaction in by PKing. So you wouldn't need the XP bonus punishing solo evils for being friendless. They could get their "bonus" by making lots of enemies instead.

This method would not be appropriate if you think solo ranking is a desirable game objective in and of itself. But I get the feeling you want it to be available for emergencies only -- when there just isn't anybody to group with. In that case, this method works well because it allows for a broader sense of PC interaction, creating multiple paths towards optimization, while more sharply discouraging lonely play.

But it sounds to me like all I have to do is send someone a tell to get my "recorded but not transacted" experience.

You can define "significant" in however you think is the most appropriate way, rather than, say, the most retarded way.

It sounds like you're asking the IMMs to hardcode a sophisticated AI judge for character interaction, which seems like an awful lot of work for marginal, if any benefit. There are enough incentives to interact in the game already with the edge system and other rewards, that a character that chooses not to interact is already shooting himself in the foot.

Yes, the system you suggested creates a massive incentive to interact. However, I don't believe such a system is either necessary or fun.