Muse's own referral announcement says it is only accepting people in the US right now. A week of r/MetaAI posts shows how many people are outside that line and what happens when they push on it. Here is what was reported, without any promises about what will work for you.
India
Two posts in the last day asked how to use Muse in India and whether anyone there had managed it. Neither had a real answer at the time of writing. If you are in India, the honest position is that Meta has not announced a launch date, and Sundog's India page will only say otherwise when there is an official source to link.
The VPN route
One user outside the US installed and activated Muse over a VPN, used it for four days, then got a not-available message. In an edit they said it came back on its own after they chatted with the agent through a connected WhatsApp account, and they did not know whether the two were related. Their own summary in the comments: Meta blocks people outside the US somehow. A VPN is a way to lose access as well as gain it, and it may breach the terms you agreed to.
The waitlist
Someone tried two email addresses and was waitlisted on both. Others asked which country they were in and whether they had tried an existing Facebook login. One reply said they waited two weeks, then signed in with a separate old account and were in immediately. Another person could not see the invite code button at all on Android.
Canada
A post celebrated Muse reaching number one in the Canadian App Store and asked how people in Canada were enjoying it. That is a chart position, not a launch announcement, so we are not marking Canada as available. It is a signal worth watching, and the Canada page will update when there is a source.
Paid but locked out
One user with an active subscription found their account restricted, could not message support because the account cannot send messages, and noticed that a brand new account worked without an invite code. If you do get in, keep the account you start with in good standing; there is no obvious recovery path yet.
What to do instead
Do not buy a code; they are free and they do not change availability. If you are somewhere Muse has not launched, reserve a place on Sundog's country alert for where you live, and we will email a code and the setup steps the day it changes, based on an official source. Meanwhile the use-case guides are worth reading, because the day-one plan is the part you can do now.