I know that the additive finite group $(Zp,+)$ of prime order $p$ when I perform the Diffie-Hellman key exchange (DHKE) protocol is insecure. I didn't however find many sources online explain why it is insecure. Neither could I find information about what possible attacks can be performed in that case.
Can anybody explain why it is insecure and what possible attacks would be possible if I would use addition instead of multiplication?
Additionally, how can I make both Alice and Bob have the same key when performing the DHKE using $(Zp, +)$?