they can be put on your minor league roster initially only if they've never played a game in the majors.
They can stay on your minor league roster indefinitely (as a free keeper) as long as they still qualify as a rookie.
You are under no obligation to call them up even if their major league affiliate calls them up.
As soon as they lose their rookie status, you have to either use a keeper spot on them, or throw them back into the pool - they can't stay on your minor league roster.
If you keep them, you lose a last round draft pick.
We use CBS, which allows you put the player on your roster so there's no need to wait for Yahoo's insufficient roster updates.