Hi there! I'm Catholic, but before becoming Catholic, I was a Freemason. Freemasonry is absolutely sinful, but probably not for the reasons you might think. There's a great deal of misinformation, rumors, and outright lies told among Catholics about Freemasonry. I hope I can help explain some of it and also help you see why it's sinful now and always.
First, it's important to know that Freemasonry is highly decentralized. Each country has its own Grand Lodge, or in the United States, each individual state has its own Grand Lodge which oversees the Freemasons within its borders. They don't really communicate with each other, sometimes get territorial and squabble, and they can't even agree on what to call each other or how the different degrees should be taught or administered. The idea that there's a grand cabal of Freemasonry controlling the levers of the world is laughable. Most lodges can't agree on a date to hold their spaghetti dinner fundraiser, no less how to drop atom bombs on Japan or manipulate currency in a global scale.
It's also important to know that American and European Freemasonry are WILDLY different. European Freemasonry definitely has a history of anti-catholic activity, though that is greatly diminished today. American Freemasonry, however, is a glorified social club for the local good ol' boys and retirees. My old Lodge, for example, met once a month, collected dues (which didn't leave the lodge except for specially noted collections which were optional), and argued about how best to donate their time to the community. We donated books to the local elementary schools library, planted flags in the local cemetery, and adopted a highway to clean. At no point is religion, any religion, ever discussed. In fact, it is forbidden to talk about religion in a Lodge. The only religious requirement of members is belief in a monotheistic God. Yahweh, Allah, Flying Spaghetti Monster-- as long as there's only one for you, nobody inquires any further.
Nothing I've descended so far is particularly sinful. There is sin, and plenty of it, mortal at that, but not in the actual activities of the Lodge. The sins are two-fold.
The first is a bit of a technically, but it's still a sin. As you progress through the degrees, you swear oaths not to reveal anything you see or hear to non-masons. You swear in God's name. There are gruesome and graphically described penalties for revealing this information, but it is widely known that the penalties are not enforced. Which means you swear to God something you know to be false. The best-case scenario here is that you're inviting God to be part of a joke. Either way, you're taking His name in vain.
The second is a bit thornier. While Freemasonry explicitly bans the teaching or discussion of religion in the Lodge, its degrees are essentially teaching a system of morality. Now, to be fair, the morality taught is judeo-Christian, but the teaching of morality is the realm of the Church as God's certified, designated teacher on Earth. You see the conflict. By teaching a system of morality, Freemasonry makes itself a religion, even if it goes out of its way to avoid talking about religion in any capacity.
None of this takes into account Freemasonry's history with the Church, which has been both subversive and violent. But while that may no longer be true (and I won't say that it's completely gone; see my above point about decentralization; it might exist in some isolated pockets in Europe), Freemasonry remains sinful and can lead to excommunication. I had to take several measures to distance myself from Freemasonry before I converted because it is an impediment.
Bottom line, don't get involved with it. There are great men who are Masons. You can be their friends, but politely decline their invitations to come to the Lodge and invite them to Mass instead.