If you are wondering who i'm inviting, it's the usual - John, Mike and Andy.

Should a dash or colon be used for the aforementioned sentence?

Hello, Eugene-- and welcome to English Forums.

Use the n-dash. Your sentence is too casual for a colon. And always capitalize the first person singular pronoun.
Hi Micawber,

Thanks for the reply.

How should i phrase the sentence to include the colon?

If you are wondering who i'm inviting, It's the usual three : John, Mike and Andy.
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Should you be curious, I am inviting three people: John, Mike and Andy.
Just a quick word from the technical writer:

The en-dasy is used bascially to mean "to" for duration or qauntities. I can't make them on my laptop, but things like April 7-9 would have the en-dash. So would something like "Somewhere about 2.5-2.7 million"

The em-dash is used as we're talking about here. It has a variety of uses, and it does tend you make your writing less formal. I overuse them, I'm afraid.

However, I do agree with Mr. M -- the dash is preferable to the colon in your original.
Yes, sorry-- I meant 'm', not 'n'.
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Since you are being informal, you could also just say
If you are wondering who I'm inviting, it's the usual. John, Mike and Andy.

Best wishes, Clive