Basically there is two parts to this:
1) Most importantly, it would be REALLY good to make it so that people’s contribution to a guild’s influence was kept track of.
It really is unfair to keep it so that you can’t tell who is an active guild member and who isn’t unless you regularly see on the chat/etc. as some people provide a lot of influence but are never noticed.
2) Give some sort of reward (like the WvW rewards) to people who contribute a lot of influence to their guild.
Obviously it only counts towards the guild you are repping etc. so if you rep and gain 2,000 influence you gain a 0.5% magic find bonus, 5,000 a different bonus, 20,000 etc. you get the idea.
This would encourage people to put more effort into helping their guild grow and reward people who do so.