The simplest way would be to use Permission Access Levels,  either via Workflow or Templates - this can give editor rights to the project but will not use the named Manager field.

If you want them added as a managers:

Add the users to a User Group
Add Workflow to Projects that fires on project creation.  
    Action 1: Set a variable ref to object,  your group name
    Action 2: New item,  run on your variable name, all members  where new object is the manager            link.  Workitem is the current object and resource teh target object
  

this would work for Projects going forward.  A CA version could be used to update Projects in-flight

Hi, 

I have the same issue and the solution you proposed works indeed for adding a new project. You did mention that we need a CA version to be used to update already existing projects. Could you please explain this a bit more? 
I have more than 100 projects for which I need to update the managers and I really would like to do this via a workflow instead of manually adapting them.

Thx upfront!

KR

Workflow would require a trigger on each project in order for it to fire - this might take time for it to work on all prpjects depending on the trigger you use.  A scheduled workflow could be used so long as the projects havent been idle to long as SWFR will skip objects that havent been updated in something like 28 days.

Dont forget a Custom action can be ran on 50 objects at once,  so using the workflow above save it as a CA,  then using the projects view

1. Filter down until you have the ones you want the managers added to,
2. Set the items per page to 50,
3. select all 
4. Run the CA  - might take a min or two to complete
5. Go to next page and rinse / repeat

make sure you check the ignore if already exisits, then even if you cant filter down exactly it wont matter if you run on projects that already have those managers 

Thanks a lot! I didn't know I could save the workflow as a custom action → problem solved !!