If by spectators you mean more than one, no. Reason for this is because I don't want the opponents to mistakenly kill someone who is not their opponent during the match. I'l allow for one spectator as a replacement for a gamekeeper, as gamekeeper's are generally there so opponents follow rules and to be a third witness to the score, so that there is no confusion when the score is final.
I know the spectators plural can try to use invisibility potion, but even in close range and in shaded areas opponents can see them, leading opponents to believe the latter is cheating. However, in the case of one spectator, I'l fully allow invisibility potion use, as there would be less confusion involved with 3 players in a server, rather than 5 or 10.