When you start the game, you click to play the map, it pulls up a menu that says survival or creative, easy normal hard etc. Under it is an option menu. Click that and make sure you trusted menu is checked properly. Otherwise it is thinking that your friend could be a griefer and wants to ruin your map. This way you can keep that from happening but if your friends is ok, they should be able to play normally.
It also might be because you are in trial mode and it has an automatic setting. Not sure.