Nowadays the Professional Services groups seems mostly used by people seeking (professional) help and not by developers offering themselves to take jobs. I also don't know how many people might seek help of a freelances for an Elgg job elsewhere (or where this might be). I afraid that specific Elgg development might not be as sought nowadays as it might have been some years ago.

If you want to get hired as freelancer for Elgg jobs, you might want to start here on the site by providing some infos about yourself and what services you can offer on your profile page, possibly also link to a website of yours where you could also display some of your working experience. And you might want to check the professional services group regularly for new posts from people seeking professional help and then reply on their postings if you think the jobs are alright for you. I just don't think that you would get enough jobs this way alone to have a full time schedule with Elgg related jobs. Maybe also check other sites where job offers for freelancers might be posted. As Elgg is a framework there might be some projects that could be realized by building up on Elgg even if this is not explicitely said so (as the person looking for a freelancer to build a "website" might not know what the best approach might be and might never had heard about Elgg even if it might fit for the project in some cases).

I don't know if it might help to get in contact with some companies that also offer Elgg services (like http://www.coldtrick.com and https://www.arckinteractive.com). They might be in need for some freelancers sometimes if they have more work than workers and both sides might benefit in the long run if you do good work. Maybe they have no jobs at hand immediatelly but they might get in contact with you at a later time once they have some additional work.