If California legalizes Marijuana will Christians and Jehovah's Witnesses be able to partake?

Only if it were possible to smoke without getting stoned. Drinking alcohol is not a sin according to the Bible - but drunkenness is. So the same would apply to any "drug". Just like cigarettes. It is not a sin to smoke a cigarette, but it can easily get to a sinful point which would be addiction to cigarettes. The chances of anyone smoking pot without getting stoned is highly unlikely (pardon the pun) so I wouldn't imagine any Christians or Jehovah's Witnesses would choose to partake if they were trying to do the Godly thing.