Recipient groups within CleverReach are completely separate from each other. There actually is no functionality within the account to join groups, but you can join recipient groups while importing or exporting data. You can simply follow these steps:
Export of data to join
First export the respective recipient groups. Make sure to only export active recipients since all unsubscribed users cannot be imported as active contacts.
Import into new/existing group
Import the data you have just exported back to CleverReach into an existing or new group. Make sure to not include the existing registration information from previous groups in this import since this is part of the export file.
Duplicates are automatically filtered upon import so that your groups are automatically joined.