Translation:Of course they will have lunch together.
Does the imperfective here means something like “from now on they will lunch together”? The compound future got pretty unclear to me, specially after so many perfective exercises. I would certainly translate the english sentence into “Конечно они пообедают вместе”. Thanks for any help here.
You're right in both of your variants. :) "Они будут обедать вместе" can be translated both as "From now on they will lunch together" and "Today they will have lunch together". And in the latter variant it's equivalent to "Они пообедают вместе". But "Они пообедают вместе" can be used only regarding one time action.
I translated "Конечно они будут обедать вместе" as "They will, of course, have lunch together", but I was told that that was wrong and the correct answer was "Of course they will have lunch together". I think these are both mean the same things and are equally correct.