Keep in mind that the walk-on player will have to be clearly better than the scholarship players at his position to get significant playing time. This is just the way the world works. It's not always fair.
In many cases the invited walk-on player is given the use of the same resources as the scholarship players. Things such as weight room usage times, training staff, dorm assignments (the walk on will pay a dorm fee (board), use of the athletic dining room (again the walk-on will pay), athletic department tutors, athletic department academic advisors among other items.
NCAA non-scholarship players do not sign a National Letter of Intent. The LOI comes into play only when scholarship monies are involved.
The same is true for NAIA and NJCAA letters of intent.
Some schools require all players to sign a "code of conduct" type of agreement, this applies to both scholarship and non-scholarship players. This agreement is a "one way" document that allows the school to terminate the players involvement with the baseball program for violations of team rules.
The signing of one of these "conduct" agreements does not prohibit a player from transferring to another school.
In the case of ALL invited walk-on players the acceptance of the initial offer to be an invitee is a verbal commitment. There are no binding written agreements involved. A player is free to sign a scholarship offer with another school after verbally agreeing to walk on at the first school. I will leave the moral and ethical debate on this issue up to the individuals and their families.
As we have discussed on this site before -- Being a walk-on player can be a great experience for some, for others it is not. I suggest that the player and his family research a school's, and the head coaches history of playing walk-ons before agreeing to do so.
In some cases it is better to get a small scholarship at a lesser baseball power or a junior college than it is to be a walk-on at a major college baseball program.
The name of the game is "PLAYING TIME", all players ultimately want to play, not sit on the bench. Being invited to walk-on makes a big difference, just be sure to do your homework.