A Hacker gathers and distributes information by illegitimately posing as an authorized source. This is all a hacker does. Sometimes the hacker impersonates a person of authority in a system, like an admin or a regular user or some such. Often a hacker impersonates routine parts of the system's own processes. Sometimes they do other things.
A society is an information system much like a computer, except that it runs on people, not software, and people are marvelously complex, free-willed, agents that throw all kinds of extra tools and obstacles to a potential Hacker. In a Medieval society, a hacker might impersonate a wide variety of people (via forged documents or otherwise) in order to get the group access to important locations. Alternatively forged order, clandestine communications, legal documents, and etc. can be used to manipulate the enemy into playing into the team's hand. A forger is an excellent medieval analog to the Hacker, especially if combined with disguise-related and other social distinctions.
As for which Merry Man best fits the role of the Hacker, that depends on which version of the tales you draw from. Friar Tuck is certainly a decent candidate, especially if portrayed more like in this show than this one. Some of the hybrid 'noblewoman, but also a badass' portrayals of Maid Marian would lend her this role, such as is done in this recent work. Really, she could fill pretty much any of the roles depending on which tale you use, though (c.f. Maid Marian and her Merry Men).
What about the Grifter?
Well, there is some similarity, but really not much more than in the normal setting. As SevenSidedDie succinctly explained, the Hacker manipulates information that people use; the Grifter manipulates emotions that people feel. Some examples follow:
Gaining the party entry to the castle on the date of a big party:
The Hacker: A forged invitation and uniforms appropriate to the attendees convinces the guards to let them in, or a requisition order (forged, of course) instructs the guard that they are here to deliver food to the kitchens.
The Grifter: Having started work a few days before, he's still 'the new guy' on the guard staff, and the other footmen are keeping an eye on him. Deliberately bumbling protocol on the party (perhaps dropping the 'invitation' in the mud or drunkenly propositioning a member of the party), the guards rush the rest of the party in to avoid an embarrassment. Alternatively, posing as a noble, he threatens to have the guard at the gate fired if the party isn't let in (despite lacking invitation) as he is a friend of King John's.
Getting a militarily superior force to back down, despite having captured the party
Hacker: This is Holy Ground! Anyone who engages in violence here or forces arrest upon another shall incur automatic excommunication from the Church! You will not violate God's Sanctuary! Stand down!
Grifter: I thought I knew you, once. We grew up together, we fought together, we took care of eachother. Well, you've got us. So are you going to strike down the last hope for a free England to escape being docked a few days wages, or are you still the honorable men I remember?
opening a door in a modern setting
Hacker: Provides fake credentials to the door, opening it.
Grifter: Provides fake credentials to the door, failing to open it. Makes a comment about how the bloody keycard never works. Asks the next guy through to open it for him. Essentially, provides fake credentials on a subconscious level to another person, who opens the door.
Calling off an ongoing manhunt in a modern setting
Hacker: Sends a series of emails to high ranking people get the hunt called off.
Grifter: Bribes the squad commander on the ground with sex to 'forget' to search a certain building.
Of course, all of these could have been done by the other roles as well. The important thing to remember about the roles is that it's more about how a character does something than what the character accomplishes. Computerized Information Technology Stuff makes it easy to see a distinction between Hacker and Grifter, but it isn't necessary for that distinction to be made.