My friends cant load into my world

We have a world that me and my friends have worked on for over a year and ever since about 5 months ago i could load into the world just fine, but i can't have my friends join into the game, and i've heard of others having the same problem and i know it isn't my connection because my connection in other games is fine. I just want know how to fix it. It started not letting people before the nether update.( Also im not sure what the affects versions means so i just put down some random ones.)