Generally, you don't, for two reasons.
First, the permissions model is not so much group oriented - you don't get at permissions via groups, you use groups when setting up permissions. There's a good reason for that, in that groups are not the only thing you can use in permissions, there are project roles, dynamic roles, users, field contents and so-on.
Secondly, since JIRA 3.6, it's better to use roles. Roles are available in all projects, can be used in permissions, but give you the huge flexiblity to allow different projects to have different access models, while still using the same set of permissions, and, that is delegated to the project administrators.
So, the general advice is:
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