From the web page:

Each Round Pen has a group and if using the website, you can go to the Round Pens link at the top and then click the name of the Round Pen to see/join the group.
Each Ministry Team also has a group and can be found under the Leadership page, click the name of the Team you would like to see/join the group.

From the app:

You'll want to find the "Chat" icon at the top right
If you are already in the group, you can see it here otherwise you can click "Discover" to find a new one:
Under "Discover" you can search for your group:

Need to manually add someone?

Inside of your group, click on the group name: 
Click on the "Members" section: 
Click on "Add People": 
If they've logged into the App before, you can search for a Name, otherwise enter a phone number or email address to send them an invite: