The short answer is yes, however it isn't a complicated process. Most US citizens with a valid passport are eligible for an ETA (Electronic Travel Authority). This is basically an E-visa which will allow you to visit Australia as a tourist for up to 90 days. The cost is A$20 ($13 US), and it is generally approved within a couple of days. You can apply at https://www.eta.homeaffairs.gov.au/ETAS3/etas
Melbourne can be reached non-stop from the west coast of the US in about 15-16 hours. A trip from the east coast will take about 21-24 hours total, so we'd recommend stopping for a night or two along the way. The easiest way to reach Melbourne is from Los Angeles - Qantas, Virgin Australia and United all fly nonstop 1-2 times a day. Generally this can be bundled with connecting flights from the east coast. (Qantas has suspended its nonstop flights from San Francisco to Melbourne due to reduced demand from Coronavirus. They may be back before August, but we'd still suggest flying via Los Angeles.) For those coming from New York, routing via Asia is also an option - flying via Tokyo, Hong Kong, or the Philippines is marginally longer, but may be an opportunity for you to experience a new destination! You can also go the "long way around" via the Middle East (Dubai, Doha or Abu Dhabi). This is generally a cheaper option, but can take as long as 35-40 hours!
We have not organised a block of rooms at a hotel. Because many of our guests are coming for a week or longer, we would suggest organising an AirBNB - Melbourne has hundreds if not thousands of AirBNBs at a reasonable price. We'd recommend staying in the City of Melbourne (CBD, Southbank, South Melbourne, Docklands, etc.) - if you need help finding some place or need advice before you book something, let us know!
Since everyone isn't staying in one central location, we have not organised transportation to the wedding venue. We'd suggest taking an Uber or a taxi. An Uber from most parts of the city to the wedding venue will take around 20 minutes and cost A$25-30 ($16-20US). The metro North Williamstown station is a 20 minute walk from the venue, but we'd recommend against this - remember it will be cold and most likely raining!
To be honest, we're not really worried about it at this stage. It's not widespread in Melbourne and will hopefully be gone by the time August comes around.
That's really kind of you! We've actually had a few people ask us this already, so we have set up an online wishing well. The "wishing well" link above will take you there!