Adding and emailing multiple reviewers

I would like to create a custom action where the user selects a group, and then all of the members of that group would be added as reviewers for the project.

Then in another custom workflow all of the reviewers would be sent an email.

I could just add a reference to the group in a custom field, but I would rather that the initial reviewers could be easily selected from a list of pre-built groups, but the the actual list of reviewers could change dynamically as the project progresses.

Thanks,

CE