Growing a community from scratch takes time and effort. You are going to need great content and offerings to keep people interested. Making it easier for your members to join your community is a good idea as well. Adding social login capability to your site is one way to do that. Nextend Google Connect for WordPress is a free plugin that lets your visitors register for your site using their Google account.
Nextend Google Connect not only lets your new visitors sign up for your community using their Google account, it also allows your previous registered users to attach their accounts to their Google profile. The plugin has multiple designs and does not require you to do any coding to get started.
This plugin is free and easy to customize. It could certainly encourage more people to join your community.