I would say not at all, except for maybe in the specific area of a park where there happens to be a "meet" going on, and probably in the area of Epcot where the Mega-Mouse Meet will be going on on Saturday. There currently are less than 2000 people registered. Now, you do not have to register in order to attend, but I don't think it is enough to cause significantly more crowding in the parks or resorts.
Pop-Warner, a much larger event that will be bringing in close to 30,000 people is much more of a concern during that time, but even that doesn't significantly affect the parks, just the value resorts (& maybe some spill over into the moderates).