If you met online, for instance in a dating site, you can apply the exact same strategy.
You keep it simple and casual.
“Hey! I was thinking of going out with some friends to this party on Saturday – I think it will be fun – I have an extra ticket and my best friend wants to meet you too… - 8 am at the square? – Works for you?”
You see again that this type of casual invitation is always fun to respond to.
If you connect well at this first date, you can always sign up for more!
However, if you feel there is no chemistry, you did not invest too much and it is very easy to pull back.