When I'm in Audience and click on the Recent Activity to see who's signed up in the last 7 days, I get a list that includes people who signed up 18 months ago. I have had people who have signed up more recent than that. This feature seems to have a bug.
I jumped in your account and can see that there are 3 contacts that display when viewing who has signed up in the past 7 days. Viewing your contacts I can see that you have more contacts that have signed up within the last 7 days. Are you using the API to bring contacts into your list in Emma? If the API is being used with a custom form on your website, this feature will not include those contacts that have recently signed up.
You can run a segment in your account to search for people based on their contact since date so that you can keep track of your new contacts. Head over to your Audience page to the segments tab and click the 'create a new segment' button. You will want to find people based on Contact Information, search for people by field Contact since and then enter in your criteria.
Take a look at this support article for more information on creating segments. Feel free to reach out here if you have any questions!