You can quickly create users in the Modx manager. I think you would need to give them the proper permissions of posting (creating new post), editing, and so on (but it depends on the solution you choose).
Then you can hand the IDs and passwords to the posting team.
I think the extra called Articles might be suitable for you, but pls wait for others with more knowledge (such as sottwell) to provide their thoughts. Some of these extras can be downloaded and installed through the manager, (package management), while some will need to be imported. Its a pretty smooth process, to be honest.
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