Copy all members of a chatter group. Click "Show All" and run the bookmarklet. See the member names in the console log.

bookmarklet

javascript:s=document.createElement('script');s.type='text/javascript';document.body.appendChild(s);s.src='https://goo.gl/8V4KtP';void(0);

chatterGroupMemberList.js

// From: http://stackoverflow.com/a/14570614/95750
var observeDOM = (function(){
    var MutationObserver = window.MutationObserver || window.WebKitMutationObserver,
        eventListenerSupported = window.addEventListener;

    return function(obj, callback){
        if( MutationObserver ){
            // define a new observer
            var obs = new MutationObserver(function(mutations, observer){
                if( mutations[0].addedNodes.length || mutations[0].removedNodes.length )
                    callback();
            });
            // have the observer observe foo for changes in children
            obs.observe( obj, { childList:true, subtree:true });
        }
        else if( eventListenerSupported ){
            obj.addEventListener('DOMNodeInserted', callback, false);
            obj.addEventListener('DOMNodeRemoved', callback, false);
        }
    }
})();

var memberNames = [];

function collectMembers() {
    var members = document.getElementsByClassName("memberDisplayName");
    for (var i = 0; i < members.length; ++i) { 
        memberNames.push(members[i].getAttribute("title"));
    }
    var nextLink = Sfdc.get(".notLastPage");
    if (!nextLink.getAttribute("style") || nextLink.getAttribute("style").indexOf("inline") != -1) {
        Sfdc.get(".notLastPage a").click();
    }
    else {
        //window.prompt("Copy to clipboard: Cmd+C/Ctrl+C, Enter", memberNames.join("\n"));
        console.log(memberNames.join("\n"));
    }
}

// Observe a specific DOM element:
observeDOM( Sfdc.get('.groupMembersContainer .content'), function(){ 
    console.log('dom changed');
    collectMembers();
});

collectMembers();