The first step to managing your site is to get some users for it. If this is an internal site, for example, for a club or business, that won't be difficult as you have a readymade audience. If it's a public site, you'll need to use social media and other methods to make people aware of the site.
The most important factor that increases the chances of someone joining a social media or community site is the people already on it; if their friends are using it or there are other users who they find interesting, they're much more likely to join. So the more users you can get, the more you will gain!
Here are some tips to help you manage your site:
Spread the word as widely as you can. Use every opportunity to tell people about your site. Think about producing cards, stickers, or other fun items with the site's URL to give people so that they won't forget about it.
Make sure you're active on the site, and others will be too. Post engaging, interesting content that's relevant...