Tell me please. There are two projects for two different companies. Now users of one project can see their own and the second project. How can you make sure that users of one company do not see another project and vice versa? Thank you.
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Jira only has "restrict" at an issue level.
The rest of it is "permit" type access. Your two sets of people have access to the two projects because the project permissions say that they can see it.
To "restrict", you don't add rules for "you can't see this", you take away the rule that says "you can see this".
So, check the projects permission schemes. Specifically the "browse project" permission. They probably say something like "all logged in users", which is letting them in.
Work out how best you want to distinguish between the two sets of users, and add a "browser" permission that uses it. I usually recommend using a role (so have a project role of "user" with browse access, and then put the right people or groups into that role in the projects separately). Once you've let them in that way, take away the permission for "all logged in users"
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You will need to create a Permission Scheme with only Roles in each Permissions
then once the Permission scheme is associated to the Project, add the roles in Projects
So users when added to Project will be able resticted
Here's a reference