I know it is still in developement.

When playing in a multiplayer server, I (The Host) can enter the portal block with no problem, text showing up, being teleported to wherever, but my brother gets a black screen and nothing else when he clicks on it after me. Another thing, when I'm in the aether itself, and my brother comes in after me, we are both put onto a loading screen that doesnt actually load into the aether, and basically we cant play the mod.

This is being removed in the port.

actually a known bug afaik, only one person can do the prologue at once, if two people enter they're both stuck until one closes the game, or if one enters after the other the second person to enter cant do anything until the first person is out, I don't remember if its been reported on here yet though

Check if:

1. the portal block is inside the protected spawn area.
2. if yes, place the portal block away from this area.
3. retry to enter in the Aether,