Request to join group

This pull request addresses #458
I am trying to edit this:
```Handlebars
<template name="groupMembersList">
  <ol class="members member-list {{#unless canModifyMembership}}read-only{{/unless}}">     
    {{#each members}}                                                              
      {{> member}}                                                                
    {{/each}}                                                                      
 </ol>                                                                        
</template>
```

I'd like to set ```{{> member extendedDataContext}}``` but at this point ```@``` is a Person, not Group instance.

Well, I'll find a way to do it eventually, but I'd like to get some hints (because I'm waisting too much time).
So now I'm trying to set up extended data context in the parent template (groupMembers) that will iterate through all members and set them appropriate flag.