The elgg core will do that by default. Create a new group, now try to create a new blog, you can see that in the access pulldown the group name will be shown and if you select the access to the articular group, only group members can see that post.

Yes, I know that. Sorry I've probably not explained my problem like I want. It's just that, my users of my elgg site want to, when they go on Pages then -> New Page, they want that in the access pulldown, ALL the groups that they belongs to appear, so that they can choose in which group they'll publish their new wiki page. It's for improve Navigation into the website and also to publish more easily, than having to go in one groupe then publish a new wiki page...

Nobody knows ?