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What you are wanting to is not really possible as the only way someone else can have access to the site is by you sharing your login details. However, this will give them access to the other sites in that account.
My best suggestion is that you create a separate account for the site you want to collaborate on and share these login details with the relevant people. This way you can keep the shared site separate from any other sites in your account.
I hope you can work with this and please let us know if there is any further clarification needed.