I've found a walkie talkie to be a very useful addition when hiking. However: This might not be the ideal solution if you're in a place where reception might be weak. I also don't think that a strap/buckle is a good idea, especiall in large crowds. You're going to annoy other people when they're getting cought in your "setup" or you might even hurt yourself. Bad idea.
No matter what you choose for staying in contact, you always should have a backup meeting point. It's not that hard remembering "If we get separated, we'll meet at XYZ at 17:00".
As a side note: I prefer not to enforce the "let's stay together" rule. In a crowded area this simply puts you under a lot of stress. You constantly check whether your party is still with you and forget enjoying the experience of just "being there". Give yourself some slack and arrange something kike "We'll try staying together but if you wanna take a look at X or get stopped by Y then we'll meet again in two hours at Z". A lot more enjoyable.
Of course this doesn't work that well with small children :)