In general, in virtually every state, you can be legally justified in using force to defend a third party if that third party meets all of the criteria for justified self defense: innocence, imminence, proportionality, reasonableness, and avoidance. If you do not know absolutely for sure that the other person has all five of those criteria, then you are assuming a high level of legal risk (not to mention physical risk) by intervening with force.But, even if one individual starts an altercation, with another individual, but the individual, who started it is on the ground.and the other guys friends jump in and he's now on the ground outnumbered, can I intervene to save his life?
As pointed out above, you can use non-deadly force to defend agains a non-deadly force attack; you may be legally justified in usng deadly force if the attack is with deadly force.
Your hypothetical incorporates one basis for deadly force: disparity of force (many against one).
However, it also incorporates a tragic assumption found in society, and reinforced by movies and tv, that the one apparently losing the fight is the victim and deserving of defensive support.
As @Kleanbore has been recommending on THR for years, please get Andrew Branca's book, Law of Self Defense (he will send it to you for the cost of shipping) and read it, or take his online course on this topic.