I'm currently running a project where a very simple Kanban setup (Backlog/Doing/Done) is working great for us—most of the time. But we're also highly dependent on a lot of other teams' output—easily 1/3 of our tasks are at least potentially blocked if those other teams don't provide their pieces. These other teams are a mix of freelancers, other groups in our company, other companies working on the same large contract, etc.
We have fairly good communication with those teams about the status of those tasks, but for various reasons, some of them don't use our tools, they don't want to, and we can't make them. (For one thing, a lot of them don't even work for our company.) We're very happy with a super-lightweight Trello setup, but I'm looking for a better way to tag tasks assigned to different people who do not have accounts in our tools.
In other words, I need to assign tasks to external groups or individuals, communicate with them on my own, and move the tasks around the process, without them needing to have accounts.
What's a lightweight way to do what I want?