Your lecturer was correct. Just follow what he said and you'll be fine. When you remember to pray, you're going to do it anyway. So just call your friends to prayer. If they are too lazy, just tell them the time you will start praying, and tell them to be ready to join you. Leave them to worry about themselves. If you made your effort, then don't force them. They will eventually feel guilty that they did not join you on time and did congregational prayer. The limit you should wait should be that they join you and be ready close to the end of the prayer time if you really want to give them a chance to join. That way, you can't be blamed, hopefully. Allah knows best. If you leave them with all the chances possible, then you did well. Remember, no one carries anyone else's burden on the Day of Resurrection.