I wonder if there is a way to designate certain users to a topic as collaborators, so they can edit the post. I am aware that the Wiki feature exists, but that would open the edit permission to all users beyond a given level - and that might not be desirable under certain circumstances. Is there a plugin that currently does similar things? Thanks!
Do you also want it to be publicly readable on the site, rather than a PM? With a PM you may add exactly who you want, including groups.
The circumstances often determine the answer, I encourage you to share your use case in as much detail as possible. 
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Thanks for your reply!
Yes, we want those posts to be publicly available. Here is the use case:
We expect users to upload “projects”, i.e. posts with special attachments (some sandboxed code that can be run in the browser). We know some users have already asked for collaboration, i.e. multiple users can edit the same “project” asynchronously. So this is different from Wiki, which everyone (within a group) can edit.
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Thanks for providing more context!
I’m going to pitch another workflow idea, not to discourage this feature request but to assist in defining the scope. 
Are the projects expected to be “finished”? Or are they continually edited in public?
If I were doing this today, I would just encourage users to create a PM for their collaborators and make that a wiki. Then, when the project is ready for showing it is either copied into a public topic, or the PM is made a public topic.
Of course that presumes a lot about how your users produce projects! But I wanted to share that the current collaborative features in Discourse support group topic writing, but there are not currently user-level permissions on public topics to exclude users from collaboration.
First, thank you for your kind response. We are trying to build an open-source community around existing open-source software too, so we appreciate your help.
While I think it is a great idea to encourage people to use PM today, it is likely that sometimes the projects must be edited after publication. For example, other users may point out potential issues or ask for more documentation. In those cases, it would be much better if multiple users besides the official author could edit the post.
I wonder how challenging this feature would be. We are also happy to contribute some code or an extension if that is not too difficult - and if someone could point us to appropriate places in the repository.
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@maiki I should have used the reply feature.