LUCY allows you to add new recipients in an existing group, which is used in a running campaign. To add a recipient to a group, you can follow the instructions here:
Add Mail Recipients

If you added new recipients within the existing campaign group LUCY will ask you if you want to send the mails immediately. If you don't click yes, the new recipients will not be added in the running campaign. If you want to send the mails immediately make sure the "send mails immediately" check is selected.

Please note: You cannot add a new group to a running campaign. In case you want to add a new group you have to stop the campaign and start it again. Make sure you don't restart it.