Share Groups Work same as Share accounts but with this all the accouts also share under that particular Group, with this you do not need to share many account to a single person.
Don't wary this is also share with only view permission so shared person not able to do any thing.
If you have sales team and every sales guy have different customer. so you can divide customer as per sales guy under a group and share that group to sales guy.
So now he have following access.
#customer vise outstanding
#customer vise sales
#customer vise purchase
#email ledger etc.