This is a discussion we have had 10,000 times in the forums. Please, make an effort to search for answers FIRST.
The group based voicemail boxes are not accessible. 3CX simply collects the recording and emails to the group. It is not a traditional voicemail box. As such, you cannot set the greeting on these "voicemail boxes".
If you need to be able to specify a greeting and connect and listen to the voicemail, then DO NOT USE the group voicemail feature. Instead, simply direct the destination on no answer to a dummy extension that has a voicemail box enabled, and set this user to forward emails to a email distribution list. There is effectively no difference in functionality and you get everything you are requesting.