There is no way for anyone other than logged in managers to see any member details, or even how many members a group has unless the manager enables the member directory.
You can choose whether to have the member directory enabled or not from 'Membership Settings':
This then allows you to determine whether members (or anyone, if the group is public) are able to view the group public profiles of members, or not.
Users with a GroupSpaces account can edit their public profiles from the 'My Groups' link in the top right of the page (which is accessed from a drop-down menu by clicking your name), then clicking 'Public Profile'.
To find out more about hiding member e-mail addresses or changing what name is displayed for your members, see our article on hiding names and addresses.