If you are worried, tell them not to do it on bank time. They can meet in the parking lot after they clock out if they want to.
We have a lottery pool that I participate in. It consists of handing the person who buys tickets $$ once a week to buy tickets on her way home. We play the same numbers all the time so we don't have to meet and discuss anything. Not a time waster by any means.
And none of us are going to embezzle to buy lottery tickets or let it take over our working lives. We realize no matter how many tickets we buy we aren't going to win.
We have won $250 twice - just enough to keep us hoping.