There are no "restrictions", Jira has a simple permissive system. If you want to stop someone from seeing or doing things in a project, you need to remove their access to do so.
A lot of us have problems with Atlassian's default "can do anything" approach, you won't be alone if you want to complain about it.
Are you working with Company Managed projects or Team Managed projects?
With regard to Company Managed projects:
Permissions for Company Managed projects are managed by Permission Schemes. The specific permission that allows users to see a project is the Browse Projects permission within the scheme.
Often the permissions are allocated to "Any logged-in user" or to the site-wide user group named "jira-sofware-users". If you want to block users from accessing the projects, you need to adjust the permission schemes for the those projects to ensure that they are not set up as "Any logged-in user", or if allocated based on user groups that the users are not members of the group(s) to which permissions are allocated.
Then, for the project(s) you want them to be able to access, you need to set up a permission scheme that does grant permissions to the new User Group you created for that group of users.
Here are a few useful links:
If you are also using Team Managed projects, permissions and access are managed differently. Refer to https://support.atlassian.com/jira-software-cloud/docs/next-gen-permissions/
If you have Team Managed projects in addition to the Company Managed projects, you will have to change the Access settings for the Team Managed projects to block access to them, assuming you are trying to grant the group access to a Company Managed project.
If you are trying to grant access for this group to just one Team Managed project and block access to Company Managed projects, you would still need to do what I described above concerning checking the Company Managed project Permission Schemes and potentially change them to block access to those projects.
For JSM June Challenge #2, share how your non-technical teams like HR, legal, marketing, finance, and beyond started using Jira Service Management! Tell us: Did they ask to start using it or...
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