This is a perfect question for 3FG, but since I haven't seen him around lately, I'll take a stab. D1 allows a maximum of 35 players on a roster. Teams can carry less than 35, and often do. The Roster is set the day before the first game in the spring.
Only 27 players can be on scholarship. Teams do not have to give scholarships to the full 27, and sometimes do not.
If a kid is on scholarship, he is for all intents and purposes on the Roster. He can still be run off during the fall, but unless the NCAA grants some sort of exception/waiver, he still counts as one of the 35. Waivers used to be dang near impossible. I have HEARD that is no longer the case, but I've been out of the loop for awhile.
Technically, a redshirt is a scholarship player on the roster who does not play enough to lose a year of eligibility. The term is often used for non-scholarship guys as well, but as I understand it, it is not technically the same thing.
The key to your question is whether the player is on the Roster. If he is, he practices with the team. If he is not, he cannot. However, like any other student, he can sometimes use the baseball facilities. If a non-scholarship player is cut in the fall, some people say he is a redshirt. He is NOT, and cannot practice with the team. Some teams have been known to say that such players are on the "practice squad." This is BS. There is no such thing under NCAA rules.
Hope this helps.