Which P2P platforms have auto-invest? Should you turn on the Auto-invest function for P2P to get in on all the deals?

You can go ahead and check their website. It is easy to find.
I know funding societies, moolah sense and minterest have them.
There are definite advantages for autoinvest. It gives you priorities on the loans you desired or preferred. It helps you secured your loans
You also do not have to waste time waiting for loans.
You can always opt out when u dislike the loans and put more if you prefer the risk-reward of the loans.
Thus, I would say it will be convenient to have this auto invest function set up

As far as I know both Funding Societies and Moolahsense have them. Turning them on gives you priority over those who subscribe/invest manually.