Hey [Friend's Name],
Hope this letter finds you well! I've been thinking, it's been way too long since we all got together, so I'm planning a little get-together and I really want you to be there!
I was thinking we could meet up at [Location] on [Date] around [Time]. We can catch up, share some laughs, and just have a great time together.
Let me know if that works for you, and if you have any other ideas for the gathering. Can't wait to see you and catch up properly!
Take care,
[Your Name]