This is a letter example to send your congratulations to your friend or coworker on his/her engagement. Wish it help you !!
Dear [name and name],
Congratulations on your engagement! After [time passed] years as a couple that I admire, I'm glad to see you guys finally tie the knot and settle down. I promise to be at the wedding. Thanks for inviting me at this truly wonderful event!
See you guys soon!