How to change the text of an email about adding a user to a group in the Members Area?

You can change the text of automated emails in the Members Area Settings → Advanced → Mail Settings.  

The following templates are available for editing:

• Adding a member via admin interface
• Adding to the group  
• Password restoring   
• Email editing   
• Password changing   

Detailed guide on configuring automated email templates: https://help.tilda.cc/membership#email