Just picked up Diablo 3 for PS4 - a few questions

Hey guys,

Just picked up Diablo 3 for the PS4 because I've got some friends coming over tonight and we need a good couch co-op to play across a few gaming nights. I last played it on 360 in like 2014 - few questions.

• We're playing couch co-op, will seasons affect us? I don't really understand them.
• Is co-op on couch split screen or do we share the same one?
• Is the necromancer live now?
• Anything else I need to know about the game in its current state? When my friends aren't around is there a decent online community to play with?