The spider in your photos is most likely the agrarian sac spider or yellow sac spider, Cheiracanthium inclusum, or the closely related longlegged sac spider, Cheiracanthium mildei, both now placed in the family Eutichuridae. It is difficult to separate these very similar species without close examination of their reproductive structures.The agrarian sac spider is native to North and Central America, while the longlegged sac spider has been introduced into the eastern US from Europe. Both species may be common in the foliage of trees and shrubs as well as herbaceous growth. They are found in urban and suburban yards, orchards, and field crops, as well as being some of the more common spiders found inside buildings. Of the two, Cheiracanthium mildei is usually more common indoors, with Cheiracanthium inclusum increasingly becoming more restricted to outdoors as it is displaced by C. mildei. They are called sac spiders, because during the day they hide in a silken sac among the foliage (webbing visible in your grape photo). They are very active at night wandering around searching for prey. Unusual for spiders, they may even sip nectar from flowers for extra energy to keep them going. Members of this genus are known to occasionally bite humans. The bites may be painful but are usually not serious. As to how the spider got into the grapes, there are several possibilities. Since these spiders live both indoors and out, this spider may have been in the grapes in the field when harvested, or one may have entered the grapes while in the store, or even in your own home if they had been sitting out for a few days, long enough for them to create the sac I have included a link below to a website with more information about these spiders.

http://ento.psu.edu/extension/factsheets/agrarian-sac-spider