Sent Nick Valentine to the Co-Op not realizing I was already there. He just kinda walked off, and now I cant find him! Is he lost forever?

Switch companions and send him to a settlement.

But I can't find him at the co-op

I sent Dogmeat to Sanctuary Hills, haven't seen him since.

For piper check detectives office at diamond city, for the dog check the dog houses and for the rest check any place you see them quest related.

I figured out why Dogmeat is so hard to find if you send him to Sanctuary,

There are tons of Doghouses and he can be sitting in any of them waiting for you rather than walking around...Oh Summer already said that,.

Build a bell in your settlement. When you ring it, NPCs go toward it, including nearby companions.