The predraft workouts are an exciting time. For most, its the first time you get to see your player hitting in a big league park. It's fun to see players from around the country work out, some with "national" reputations. At every workout my son attended, he knew a number of players, whether the workout was on the east coast or the west coast. It's amazing how small a world the baseball community is.
I would say most players got drafted by somebody, but of course not all get signed.(High school players)
Some players get notified early that teams would like to have them work out, others are lucky to get notified a week in advance. Usually their area scout will notify them. Some teams will actually pay for a player's expenses, some will pay expenses only if they draft & sign your player and others won't pay for any expenses.
Workouts are are held within a short period of time, causing scheduling problems because it's not unusual to have 2 or more clubs have workouts on the same day. The number of players that attended varied with as few as 10 to 30. My son and I had a great time during these workouts. It was a "senior trip" he will never forget. Good luck to all.