Check the Active Users page You'll see some users in red who may not have been welcomed before, or have made very few changes which would indicate that they might be new.
Check the recent changes. If there is a new user, it will say as an edit summary: (New user account). You can then click on the red linked talk page and then greet them. This is also helpful if you are looking for newbies, vandalism, etc.
As you are working in the Wiki, there are two additional places where you might find new users:
Article History Pages: If edits were made by users with red Talk pages, they have not yet been welcomed.
Discussion pages: On these pages, new users will often not know to sign their comments, so you can look on the History page for who made the edit. If they did sign their comment, and their username is in red, check to see if anyone has welcomed them yet.
Keep an eye out for new users in all of these places, and welcome them when you see them.
There are many users without any edits. It is widely accepted (but not required!) that users with no edits should not be welcomed to save on server resources and also because most templates start with "Thank you for your Contributions" (or similar).