Hello!
How can i change the text of the Invitation email?
Topic on Extension talk:InviteSignup
You should create the page MediaWiki:is-emailbody on your own wiki and define your desired email body text on that page.
You can also do this to change the email subject using MediaWiki:is-emailsubj on your on wiki.
For more information on this method, the page Help:System_message#Overriding_messages_on-wiki should help.