I and a few others created a service (a website that people can subscribe to for content) a while back as a hobby with no agreements and no structure. Now though, I am thinking about taking it further and forming an LLC for it, owned by me. Would those who worked on it with my originally have any legal claims to anything?
Yes, they have copyrights
If there's "no agreements and no structure", then every single line of code and text content and artwork (such as logos) that's created by someone else for that service is owned by them and protected by copyright, and they have exclusive rights on it so you (and your LLC) are not allowed to use that without their explicit permission.
If they don't care now, they might start caring after your LLC starts using that service and becomes financially successful (no matter how many years it takes), and they could sue for copyright infringement during all these years, causing all kinds of penalties that can destroy your company at that point.
The safe way to go is either to 'go clean' - i.e. not use anything from that service and reimplement it from scratch, or obtain a contract that they're selling all these assets for some nominal sum; if it was an unstructured hobby project that went nowhere, then it's plausible that others might be willing to sign it away to you (or your LLC) for a few beers or something like that.