I want a friend to upload some photos to my dropbox account. She herself hasn’t got dropbox, as far as I know. When I try to share a folder in the dropbox web interface, there’s a field called “invite user to this folder”. If I leave it blank, a message pops up saying “please insert valid name or email address”. Does this have to be a user with a dropbox account or can I insert any email address there? What happens after I put in her email address and share the folder, will she be alerted via email? And will this folder be password protected and only shared with her?
I think when you share a Dropbox folder, they can download from it. But they can’t upload to it unless they also have Dropbox.
Yes, you can send the link to people without a Dropbox account. The link they receive will take them to a page on dropbox.com which will allow them to download the file.
But will they also be able to upload files to my folder? That’s what I’m after.
No. You can only upload files to Dropbox if you have an account; once she has one, you can share your folder with her and you can both upload to it. As it is, if you send her a link she can only download.
Dropbox is free, why not have her set up an account?
Yeah sure, I even could raise my storage if I invite her, but I wanted to cause as little inconvenience as possible and don’t know if she wants to set up one more of all those accounts we all know we have too much of and don’t know how to keep track of all the passwords… 
I know nothing of Dropbox, but it is possible to have several dropboxes on the same computer.
You could start a special Dropbox with another email address, and then give her the same email address and password.
There are plenty of options for non-users to upload to your Dropbox via plugins. A few years ago, we had our whole college batch send us their photos for a reunion we were organising. I used http://dropitto.me (drop it to me) back then, there seem to be better options now (filesize restrictions vary among free versions of various plugins.
Just Google “allow anyone to upload to dropbox”. A few results I got are entouragebox (https://www.entouragebox.com), supply drops (http://supplydrops.com), sendtomycloud (https://sendtomycloud.com) etc.
The process is pretty simple. You register for any of the plugins, authorise access to the plugin in your Dropbox and you get a web interface with a unique address for uploading files. The plugin I used needs you to set a password that you share with the sender (along with the unique web address), I’m not sure if the other plugins need passwords too.