How can I get people to access an account, and to edit a file on my account?

I'm more confused and I think it's my fault for not explaining things properly. Also, I have no idea where the title to my post came from. I didn't write that.

#1 - I set up a folder called "stuff" I share by adding people. I add user A, B, C to "can read". I add user D to "can edit". Now any file I dump in to "Stuff" user A, B & C can read but only user D can edit. Correct?

#2 - Can user A, who only has read access, upload a file to "Stuff"? If they can, do they have edit access because they uploaded it or only view access? If only view, is there a way to give user A edit access to that document while it is in the "stuff" folder?

#3 - When I add people, the default setting is "can edit". Is there a way to change the default to 'can view"? Since the majority of the people access these files will only be viewing them. I don't want to take the chance of forgetting to change that when I add someone new.

#4 - I've got the settings on the folder done, I've added some files. So I send out an email to users, A, B,. C, & D. None of them have been on Dropbox before. What do they need to do to view the files? Do they have to set up an account? Does the system recognize their email address and let them in?

I guess that's all for now. Thank you for putting up with me.