In these cases, I decided on the shortest path, or which members came to mind first. A opening up for band B at a concert. On the other hand, links between two different musicians ARE allowed to be made if they never played together, BUT played for the same band. For example, David Lee Roth sang for Van Halen, but was later replaced by Sammy Hagar. Maybe these two people never even met each other, I don't know.
Session musicians are valid connections.
For those who don't know, there are musicians that are hired just to perform live or record with an artist, but they're not strictly committed to that band. This situation is rare, but it can still happen. If you want to know when a particular person was in Black Sabbath, read the next section. If somebody only made a guest appearance in Black Sabbath, like Ice-T or Rick Wakemen, then I'll usually list exactly what they did. Please go there before you email me questions on this topic.