In 2 Corinthians 5:20, Paul tells the believers in Corinth that they are ambassadors of Christ. That they are to conduct themselves in a manner worthy of the gospel and people through whom God makes the appeal to be reconciled to Himself.
The Embassy is a group that happens on alternate weeks where the teenagers get together to study the Bible and discuss life together. They will either be going through a book of the Bible or discussing how the gospel applies to different areas of life.
The weeks that the Embassy isn't on the teenagers stay in the service. The weeks that The Embassy is on they leave with Kids' Church during the service.