IF I create a prezi using an enjoy account ( private ) and invite using an email address to share is the person I share with required to be on wifi to present/view or can they download and save to present without wifi?
if I understand you correctly, initially yes they will need an internet connection to download and view the Prezi or in fact use it in their own account.
if you send someone an email link to view the Prezi they are pulling the data down from Prezi servers to view it
if you send then a link to edit then it is very similar - however with their own account they could exit from editing the Prezi and ‘save a copy’ of the Prezi and it will remain in their account afterwards - they can then present it this way. But in both cases they will need the internet (dont forget if you have a youtube / vimeo embeded in the Prezi you will also need to be online to stream its content).
However, if you download and use a portable Prezi you can send it to them using a file sharing service (Dropbox, Wetransfer etc) and they can present offline using the built in player on both Mac and PC. these are quite large files and so email is not suitable. Once again if you have a youtube / vimeo embeded in the Prezi you will also need to be online to stream its content)