project -> setting->internal user -> you can add users here

Hi Ileshkumar,

I'm facing the same problem as Jaime and I agree the terminology is very confusing. Also the solution you propose with Action -> Share Project does NOT work (at least in Odoo 15) when Visibility is set to "Invited employees" because the share project popup shows a yellow warning message "The project cannot be shared with the recipient(s) because the privacy of the project is too restricted. Set the privacy to 'Visible by following customers' in order to make it accessible by the recipient(s)." and there is no button to save / invite any person on that popup.

Also the warning message uses "privacy" term but there is no "privacy" setting when editing a Project, there's only "Visibility" setting so it's unclear where the privacy setting is. Does anyone have a solution here?

You need to go to edit the project
then go to the very right top corner where there is a little person icon and click "add followers" 

this is how to add employees to a project that has limited access.
It doesn't make any sense, it's not intuitive, it's a very good short description of how things on Odoo sometimes are extremely unintiuitive, redundant and don't make any sense. 
I still don't understand what is the collaborators tab

in the project settings, 
"Visibility -> Invited employees"

In reality
"Followers -> Add Followers" 

Keep terminology consistent in order to make sense and not make people waste tons of time trying to figure something simple out :$

Hi Jaime,

You will need to share this project with employees to whom you want to give access.
You can do it from "Action" of that project.
Please check below screenshot.