There is a project (AI Horde) that allows you to donate compute for inference. I’m not sure why the same doesn’t exist for training. I think the RAM/VRAM requirements just can’t be lowered/split.
Another way to contribute is by helping with training data. LAION, which created the dataset behind Stable Diffusion, is a volunteer effort. Stable Diffusion itself was developed at a tax-funded public university in Germany. However, the cost of the processing for training, etc. was covered by a single rich guy.