It's actually ridiculously easy - just grant the group edit permission on the group document. brain[sic]

The first thing you ask, which is about modifying the group template so that a newly-created group has edit access to it, is also fairly simple: you add an XWiki.XWikiRights object to the document's form. First you get an XWikiClass object by calling $xwiki.getDocument("XWiki.XWikiRights").xWikiClass, then create an instance of that class by calling its newObject() method. You set the fields in that object to the values needed - the "groups" field to the new group name, the "allow" field to allow, and the "levels" field to "edit". Do a displayHidden on that object within the form so that it's a part of the form, and thus when the page is saved it will be added to the page. To create a group automatically when creating a space is somewhat different. To the best of my knowledge, scripting XWiki, to the extent that it involves sequences of operations, is not straightforward, because each modification to a page requires a separate HTTP request/response cycle. In this case, creating a space (which, for purposes of establishing permissions, means creating its WebPreferences page and adding one or more XWiki.XWikiRights objects) is one request, and the creation of the group is another. It's entirely possible, of course, to create a form that creates the <newSpace>.WebPreferences page and adds the rights objects to it, but the name of the group would have to be input into that form, and on success, its response should redirect to the group creation page, where you would be able to add members to it. Because the creation of a group (which is a document creation operation) is a POST method, it can't be a simple redirect, but that's okay because you'll want to add members to the group anyway. You can arrange to have the form redirect to the group creation page by setting the value of its hidden field "xredirect" to the desired URL. Hope this is enough clue to get you started. brain[sic]