Adding users to edit your project plan

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I have me and two peers who work for the same organization. We have the same domain in our emails (fname.lname@companyname.com). I have a Pro account and I shared my project with these two individuals. I used their email addresses and assigned them editing rights. They cannot edit anything (the ability to add a task, edit a task or change anything on the project). They do NOT need to "create" anything. Just edit. 

I read that external users cannot edit. Depends on what is an external user. Are my peers external? Do I have to upgrade to business or enterprise? I just need less than 10 people who can edit the project (or at least participate in adding, changing, deleting tasks, plugging in dependencies, duration, etc. all the simple stuff.

What do I need to do in order to have this capability?

I sent in a request for support... dead air. Still in a pending state over a week later. Does anyone know the answer? I'm the only one using Smartsheets to run a project. Just me. All others are looking at my project plan and participating, but have no need to create anything else. Just edit.

So the question is:

1) How can a non-licensed user participate on a project plan?

2) Do I need to have a certain License level to enable #1 to happen?

3) What license level will make that work?

4) Is there anything else that would be helpful that I'm not asking in relation to this?

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