Recommend a Freelancer Collab App with Privacy

Hey guys,
I work with a group of freelancers and I post them tasks that they can choose whether to participate in or not. Right now I am doing it manually sending the info manually to each one but it’s very tedious and inefficient. I thought of inviting all of them to a slack channel or something but the thing is the nature of what we do requires it that they don’t see each other and if I put them in slack they can see each other even if they are in separate channels. So I am wondering how can I send messages to all of them efficiently and keeping the privacy intact. So far I thought of inviting them on slack as single channel guests and then using Magnify app to send them the messages but it sounds stupid. Alternatively I am considering a mailing list too but emails suck for back and forth replies and clarifications. Is there any setup that can achieve what I need. Maybe a lateral approach could also work - e.g. some kanban setup that they can see and reply on privately or w/e. Any ideas?