[What I can't fathom is why in the investigation regarding Oscar, they wouldn't have at that point then included Winston on it when they realized that they had to dig (though of course they really didn't have to) and that the investigation was going to perhaps be more than they thought. You'd think Ray would inform him of what was going on and asked him to join in, since it seemed like he and Winston were good buds.[/QUOTE]
They where investigating the occurrence with Oscar in an unofficial capacity after an informal request from Dana to Egon, Egon drafted in Ray as those two are the real boffins, Peter forced his way in, thus ending up being just the three of them with Dana at the start and running into the street. The court room scene also leads us to believe that they can no longer use their equipment or operate as The Ghostbusters officially as a court order prohibits them from performing paranormal investigation or extermination. There where only three packs in the courthouse, as these where confiscated from their vehicle located at the scene.
(what they where doing on 5th avenue, digging up the street was illegal, even more so because they are not allowed, maybe Winston wanted no part of that, being a bit saner then the rest of them, or in their rush to uncover what was going on just neglected to tell him, he was only staff in the first film, while those three where close friends who got up to shenanigans) Winston is obviously still in communication with them as shown at the very start with him and Ray at the children's party, but we never find out what Winston is doing to make ends meet. Egon is doing behavioural something in a lab, Ray owns a book shop and Peter is a crappy cable channel host. Its not till the montage after the court house that we see all four of them together for the first time now wearing all blue boiler suits. In GBII you actually see Peter being removed more from the trio and Winston taking his place. You see much more of him in the second film, he saves them from the photographic lab, he goes underground into the river of Slime etc, Which is why he ends up on all the promotional material for the second film along with Weaver.