Thank you Rob, you are correct.

I followed the advice in your reply and it works as expected.

Kind regards,
Ian

Yes there is.
Apparently you have created a customised user group named "family". I understand that users in this group have also been assigned to the "registered" group, i.e. the standard Joomla! group.
As long as your customised group "family" is not included (see basic options/included customised groups), the accounts assigned to this group will not be pruned.
So, by default install, the plug-in is already doing what you are looking for, that is if I understand you correctly.

Operate the plug-in in "test mode" (basic options, set test mode to YES) and you will see which accounts will be pruned (without any accounts actually being pruned). You may need to set the time limits extremely low (say 1, 2 and 3 hours) to see recently added user accounts. Do not forget to double-check these time limits before you set the test mode to NO.

In summary and for Premium and Business edition only:
- All standard Joomla! front and back-end groups (except Super Users) are pruned by default.
- Customised user groups (i.e. created by you) are not pruned by default.
- Customised groups can be included to be pruned.
- Individual users can be included and excluded by user ID.

Only the Business edition has the additional option to exclude a 'standard Joomla! user group' as a whole.

I hope this info is helpful to you.
Please also refer to the documentation which is part of the downloaded package.

Is there a way to exclude an entire user group from processing instead of using individual user IDs?

I have a website where all registered users are automatically assigned to the "registered" group, but in addition family members are assigned to a "family" group as well.

I want to exclude family members from being pruned, but want normal registered users to be pruned if necessary.

Thanks,
Ian