I can't upload content to a shared folder, because my account is full.

Hey juicyandlucy, thanks for posting here today! 

Keep in mind that, when joining a shared folder, you basically add it to your Dropbox account, which is why it takes up space from it like any other personal file would. You can read more about this in this Help Center article. 

Now, if you mostly need to upload content to the folder in question, the owner can send you a File Request instead, so that you don’t have to add it to your Dropbox account at all. 

I hope this clarifies!