You need two or more people, a Nether Portal, and a bucket of water.

One person go into the Nether the other one stays behind with the water.

Then the one who stayed behind puts water where you have to walk to go to the Nether. On the other side (the one in the Nether), the Portal will still be activated.

The one in the Nether must walk and go back to the Over World (be sure the water is taken away). To be able to go back, the server must spawn another Nether Portal.

This is for those who want multiple Portals. I actually found this on accident because I tried to "ban" one of my friends to the Nether.