Just managed to work on a solution for this one.
For reference, I’ve made a solution public in this editor here. Everything you need will be on the project page.
First, here’s how the database is structured.
Database
Project
- Project name - text field
Task
- Project - project
- Task name - text
- Users needed - list of users
User
- Name
- Projects - list of projects
First, on your project page, add a repeating group to the left and have it search for a list of all the users whose ‘project’ contains the current pages project.
At this point, you’ll also want to sort this repeating group by their names in a descending order.
Next, you’ll add the repeating group that searches for tasks where the project equals the current page project.
Within that, you’ll add a group (in red) and change its type of content to be a task, and its data source to be the current cells task.

Now, within this group, you’ll add a nested repeating group with a user data type. Update its data source to search for users whose ‘projects’ contains the current pages project.
You’ll also want to sort this list by names in a descending order.
Within this repeating group, I’ve added a button with a cross as the default text.
From here, add a condition onto this text to recognise when the parent groups task, ‘users-needed’ contains the current cells user.
If this condition is true, we’ll update the text to display a tick.

Now finally, I’ve just added a dropdown within the task repeating group to allow me to select one user at a time who is required for this task. You could easily change the UX of this using a popup element or something similar.
The dropdown menu searches for all of the users whose current ‘projects’ contains the current pages projects.

When the ‘add as needed’ button is clicked, the workflow will need to make changes to a thing.
The thing you’ll want to change is the current cells task. Change the ‘users needed’ to now include the dropdown value.

This should do the job. The board in my editor is customisable like this.
Hope this helps. Let me know if you need anything else.