Hello. This is my first post here. I hope I can be of help.
Anyway, I believe only holding down ABC works to pull off the team member assist function. It works for either a prolonged grab (like Joe's knee one where he repeated hits you), or for when your character is dizzy.
Pre-KOF 98, the only requirement was to have a non-defeated team member in the background. KOF 98 added the whole idea of character relationships to whether they help you or not, and whether super stocks carry over, diminish, or increase by one. The relationships were more or less story-based.
The Dreamcast conversion of 98, Dreammatch changed this slightly. Making the character relationships changable. In that version, it records what characters are teamed up the most with and calculates relationships that way. It should start like the arcade version, but I can't remember since it's been forever since I played KOF 99 Dreammatch without loading from a save. So if Kim is used with orochi-based characters all the time, picking the same chars will have even Kim like them.