Can you join the necromancer guild in oblivion

No, you can't join the Necromancer Guild because they are the enemy. You can wear necromantic robes and use Conjuration spells and act as a necromancer, but in the end you'll keep being the "good guy" and help the Mages Guild.

However, you can download multiple mods that let you join it. This is not official though. It's a pity that Bethesda didn't release an update that let's you join.