Why to do it

Imagine you were suddenly transported to an alien planet (that had oxygen in the atmosphere) and you were surrounded by weird people speaking a weird language (LOL, BRB, AFK they shout!) How would you feel? Now, imagine that scenario but add on a bit about some nice person walking up to welcome you and ask if you had any questions.

It's not the greatest analogy, but it gives you a little of the feeling someone new to your community might have. Not everyone new to your site will be new to the Internet, but they will be new to the written and unwritten rules that your community follows.

How to do it

You don't want to come off as bossy or creepy, thrashing someone new for breaking a rule. Instead, give them a friendly welcome to the site and invite any questions they might have. Be sure to let them know you're not an official spokesperson for the site.

If the admin or moderators at a website ask you to stop, you should probably stop. You might also ask yourself why they would want you to stop. Is it not the type of community you thought it was? Or are you doing it wrong? Private messages or email communication to straighten out things like this is probably the best idea.

Where to do it

Don't try to cover too much online territory. Depending on how much time you have to devote to your welcome committee mission, you might keep the number of forums or websites you do this at to one or two at the most.
If you add too many, you can soon become overwhelmed with the work. Worst case scenario you begin to become jaded, coming off as insincere in your welcoming (one of the worst things) or you tire of it all together and stop - something bad for the community as a whole.

When to do it

When to step-up and take on a greeter and helper role at a website varies for different people and different websites. Some Type A personalities might dive right in and just start doing it. Others might contact the admin or a moderator to get their guidance.

Whatever the reason, if you've ever thought about how you could help your online community be a better place, there's no time like the present to get started. If you're really good at it, there's no telling where you might go online.

Next Steps

Being an unofficial greeter on a website and helping newbies as much as you can, there's a bit chance you might be asked to become a moderator, someone with power to delete posts or punish users. No one wants to abuse the power, but it's a good feeling to be able to help an online community you care about.

While it varies from site to site (depending on the personality of the site owner), after three to six months of stepping up and helping welcome new members to a website, you're sure to start to get some recognition and maybe some more responsibility by those in charge. Who knows, maybe someday you'll be so good you'll be able to make a career of it.