I recently address that question here on this forum. Please be careful as there are a lot of scammers out there just waiting to scam you of your hard-earned money.
My advice to you or anybody on this forum who doesn’t know what bitcoin is and is being invited by friends or family to (as you say) join in bitcoin, don’t join in anything. Bitcoin is not something to join; bitcoin is simply money; digital money.
The same way people use physical money and create scam programs, invite you to join, and scam you of your physical money, is the same thing they are using bitcoin (digital money) to do.
They are giving bitcoin a bad name by using the name 'Bitcoin' as a front to scam people of their paper or physical money. Most of the time the people who invite you are themselves not aware of the snare they themselves are getting into.
However, bitcoin is real and it is the best store of value existing in the world today. The best way to invest in bitcoin is to purchase it and hold on to it without trading or spending it.
The same way you can trade in the physical currency you can also trade in bitcoin, but it is much better to purchase and hold on to it as long as possible, just like Elon Musk of Tesla, Michael Saylor of MicroStrategy, Barry Silbert of Digital Currency Group and others are doing. These guys and not trading or sell their bitcoins and are amassing millions just by holding their bitcoins.
To find out what is bitcoin you can go to YouTube and type 'what bitcoin is' into the search box.
Bitcoin is just the leading cryptocurrency; there are others. Hope this helps.