Build it and they will come - include the date in the title and ask for a sticky. For a dinner, pick a date about 2 weeks in front of you or if just for coffee, a week is generally enough
If you only get 3-4 people or 10, either way it is a success, a beginning.
Otherwise you'll talk about for weeks and there will always be someone that can't make it that day or time.
Good luck with it and have fun!