The store can not create FTP accounts, that is something you will need to do manually on the hosting provider you are using. You can setup FTP accounts for your contributors there. Contact your hosting for more details on how to do this if you can't find an area in your hostings control panel to do this.
Directories they should upload to:
Each contributor that signs up to your store will have his/her own directory in which they can upload photos to. The direcory structure is /assets/contributors/contrXXXXX where XXXXX is the ID number of the contributor. For example assets/contributors/contr00001 would be ID of 1. You can view ID numbers in your store manager user -> members area. That way you can setup an FTP account for that contributor and just allow them access to that folder (contrXXXXX) that they need.
The contrXXXXX folder may not exists, you may have to manually create it, the store will only create it if the member attempts to upload photos via the stores interface (java/flash upload / import).
If you have version 4.1.6 or less, when doing FTP uploads for contributors there will be not thumbs/samples generated for the images. The contributors will have to come back and attach the thumbs and samples to that image.
If you have 4.1.7 on up you will no longer have to worry about this, the store will auto create thumbs and sampls for those contributors that upload via FTP.