Yes! Any Friends or Share Groups that you share your folders with and give Edit Rights to will be able to upload or download files to and from folders in your account. If you do not give editing rights to your friends but only share the folder, then your friends will not be able to upload or download files from your folders.
To allow others to upload and download files to and from your folders, first you will need to share and give edit rights to your folders. Follow the steps in the image below:
Once you have shared and given edit rights to your friends, you can then copy the link to your shared folder and give it to your friends. When you give them this link, your friends will see a screen that looks like the one below. As you can see, your friends will be provided with several options for uploading and download.