While I am not sure a knowledge of Latin goes hand in hand with necromancy, you can find many free materials for learning Latin from the Learn Latin link here at Textkit. If you had group lessons over the internet in mind, then you could try a Google search for "Latin study group". You could also sign up for a class at a local college. Keep in mind that Latin is a difficult subject, if you consider it to be a small step on your way to necromastery you may be in for a bit of a surprise.
Good luck and vale bene.
flebile nescio quid queritur lyra, flebile lingua murmurat exanimis, respondent flebile ripae