how to create a project group and how to add users to the project group and also how to give permissions for the users??
We have a couple of demo videos that might help explain these features. They are from version 11 so things might look a little different in version 12. Could you view them and clarify your questions?
This video (33 minutes) covers the Project Management module. There is an explanation of team lead assignments around 16 minutes and an explanation of activity assignments around 19 minutes.
This video gives a basic explanation of permissions in the context of a test plan. Item permissions can be changed for all items of that type under the TOC > Administration > ItemTypes. Search for, in your case, Project, edit the ItemType, go to the Permissions tab, select the existing permission and edit the Identities (and Identity Groups) listed there to what you need.
You can find more information about Permissions (and Projects) in the software help file by clicking the user menu in the upper right and selecting Help. This will open the Just Ask Innovator guide, where you can search for “permissions”. There are several entries based on the context of the answer.