Well it would have been redundant to mention Marco if they wouldn't be able to find him, so it's safe to assume that Nekomamushi's guess of where he is now, is correct. From that point on it will depend on how much Luffy impressed Marco. Pirates like Marco and Vista are really not the type of guys who will change ship easily. Their loyalty to their captain, even if he's dead, is tremendous. They have followed WB for decades after all, however they were not able to avoid WB getting killed. They were not able to avoid Ace getting killed. After that they failed in taking down BB and getting revenge for Thatch, WB and Ace. It's really no surprise Marco went off the grid and that people now speak of the former WB Pirates and not any longer of the remnants of the WB Pirates, indicating that the crew disbanded (though this might just be a difference in translation). In the end what unified them was the person of WB, without him there's nothing left to keep them together.
Now if you add all of that together, well I reckon Marco has no idea anymore what to do. He failed at everything, so he went into hiding because there's no alternative. There might be other former WB pirates with him, he might be alone, but the fact remains that it will not be an easy task to convince him to come out of hiding. What purpose would it serve? It wouldn't be much of a story either if Marco would oblige that easily and I would guess that Luffy will somehow be crucial in convincing him. As I said a pirate the caliber of Marco is not someone that's going to just change the captain he serves out of pragmatic reasons and Luffy is way younger than him. Normally this would be unthinkable, however on the other hand what Marco did succeed in, was saving Luffy during the War of the Best. Luffy had been acknowledged by WB himself and Ace firmly believed he would be able to become the PK. What's more Jinbei already befriended Luffy and now Luffy is befriended in the Minks. Luffy also has a grudges against BB, which makes it interesting for Marco to support him as Luffy would need to confront BB sooner or later.
So yeah the chances that Marco joins Luffy aren't that bad, but it will all depend on how strong Luffy's impression on Marco is. Luffy needs to be able to install a certain believe into Marco, a believe that makes him truly think Luffy can do what he says.