Hi there, wrong forum I'm afraid.
Here are your options in the osu forum:
A: Beatmap projects
If you want to join an ongoing set you're best bet would be the
Beatmap Projects forum (Link).
There you can see all the beatmaps that are looking for GDs, although most of them are std (osu mode) requests.
B: Modding Queues
If you just wanna offer to make a Guest Diff, I highly suggest to make a GD queue in the
Modding Queues forum (Link), specifically for you would be the
Mania Modding queues subforum (Link).
There you offer to create GDs, you can also includes rules or checklists for potential requesters to follow.
Most of these forum's are Modding queues, but a lot of them also offer GDs, you could take a look at those to get an idea of how to make your queue.
If you need help with any of the above options, or have general questions regarding this topic let me know.