This is a known issue and MijuGames have taken steps to fix it, but it does still randomly occur unfortunately. Some things to try are:

1: Make sure the host player goes through the portal first and alone. Once the host has checked the door in the wreck is open then any guest players can enter the portal.

2: If a door is still either closed or only half-open, go back up the ladder and come down to the door again. This may take a few attempts but the door should eventually open slowly. The host AND each guest player may all have to do this.

3: If the door refuses to open, have the host player carefully use their jetpack and the F4 'unstuck' key to get on top of the wreck. If the wreck is intact next have the host player re-enter the wreck beyond the door using their jetpack and F4 and then have them put a fusion energy cell in the wrecks reactor. This may help get the door open for guest players.

If the host player finds that the wreck is obviously broken or missing beyond the closed door, they should use the ladder to return to the surface.