Share Groups works the same way as share accounts but with Share Group all the accounts under that particular Group are also shared.
Not to worry as this too is shared with only view permission, so the person with whom the group is shared won't be able to do any thing.
If you have sales team and every sales guy in your team have different customers, you can divide customer's group as per a sales guy under the group and share that group to sales guy.
He would have the following access to the following details and options:
#customer vise outstanding
#customer vise sales
#customer vise purchase
#email ledger etc.