If you mean users who are not active, then look at the last login times in the user browser (and cross check with their activity streams).
If you mean the users with the "inactive" flag set (so that they cannot log in or use JIRA), then it's also indicated in the user lists.
There's no such thing as an "inactive group" in JIRA - a group is nothing more than a set of users. You can't deactivate groups, as they're just sets of users. If you mean you want to think about deleting empty groups, then the list of groups shows the number of members. Finding out if a group is used in JIRA is a bit more difficult though - you have to look through fields, workflows, project roles and the rest (although a well looked after JIRA will only use them in project roles)
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