There is not much that you cannot do with the REST API. Documentation here: http://docs.elgg.org/wiki/REST_API

If you are going to use it, make sure you are using Elgg 1.7 as it fixes a lot of bugs. You may be interested in reading this: http://elggnews.com/pg/blog/cash/read/120/new-and-improved-web-services-api-in-elgg-17

@Cash

Thanks for the quick answer. Although I'm afraid I've no time to migrate to 1.7

I will try to do it in 1.5

Regards

The REST API in 1.5 was very buggy!

Maybe, but I have no time to migrate versions...

I just need to expose users and their usernames.

Do you think I would be able to do it with 1.5??

Thanks for the interest!

even i'm looking around for some guides to expose some specific entities. I would really appreciate if anyone can post a sample tutorial to retrieve the "threads posts" in a discussion group. 

thanks

Yeah, I would like to expose user entities for instance.

I guess I should return an array of users and usernames, is that right?