Before you proceed, you need Spaces installed on your phone.
How to add members without saving their mobile number in your phone?
Step 1 - Open the Space in which you want to add members and Navigate to the Members Tab.
Step 2 - Click on Add new member
Step 3 - Click on Add new.
Step 4 - Fill in the contact details of the member that you want to add and click on Add.
Step 5 - You will see those contact details selected and then click on the top right tick corner icon.
You are done!