Creating a user-operated blogging site

I'm trying to find the best setup for a website I'm building, and I keep running into problems.

I've been tasked with creating a portfolio website for an arts collective. Every month they set a creative task and then the members of the group respond in whatever way they want.

Ideally I need to make a website that the users can upload their responses to quite simply and then the website will categorise them into the correct spaces.

At the moment I have a home page with a portfolio displaying the different 'months' as projects within that portfolio. And ideally I'd like my clients to be able to submit blog entries to go into those projects, and if a project doesn't exist they can create a new one.

Has anyone got any tips on how to get a setup like this to work, even if it's with some compromises?

Well, Squarespace has contributors option. You can set up folks with permissions level to access the site with their own username and password. You invite them to be a contributor and you set the level of what that can and cannot do. The second option is Basic Authors. You post the content they send you, but there is a built in list of authors you create and just attach that authors profile to the post when you make it. 

Here is the help for both ways: https://support.squarespace.com/hc/en-us/articles/206537297-Squarespace-permissions-explained