Before assign a task to a person, you need to add them to the project and a board (if it's private to a project). As there can be different departments in your company and each of them has its own working project and boards. Let’s say there’s no need for an HR-specialist to be on Web Development board.

So once you have invited all members to the company next step is to add them to the project:

     2. Open Members tab > Add Members

     3. Open a board, go to its Menu 

    4. Add members

    5. After that you can add a person to the task
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