Well, personally I don't think that's the best idea. It's a one-sided arrangement since it would give you more exposure in terms of the visibility of your reviews, but by not joining that community you're not supporting them or helping to endorse their community. I think that it's essential that the communities are mutually advantageous and provide people with an equal opportunity for success on the site. As reviewers, we offer our ideas, opinions, and any information we may have in order to provide food for thought and as community founders, we try to take those reviews and expose them to a wider range of people. If someone was able to post a review within a community without first becoming a member of that community, then it would (1) make it harder for community founders to attract new members and (2) it wouldn't be fair to those people who are greatly invested in that particular community and have made numerous contributions.