PARAGRAPH 1: reason for writing (to invite)
I'm writing to invite you to...
I'd love it if you could come to...
We're organising a.... and would love it if you could come.
PARAGRAPH 2: details/directions
In case you don't know the way, I'll give you some directions.
Here are a few directions so you don't get lost.
FINAL PARAGRAPH: closing remarks
Look forward to seeing you then.
I hope you'll be able to make it.
Please let me know as soon as possible if you are able to come.