The Only One
This is my interpretation on it using PHB, DMG, and Manual of the Planes:
You cast it, and can move 50 miles per hour. Manual of the Planes says 42 mph, but PHB supersedes it. However, due to the nature of the Shadow Plane, all movement is 50 miles per hour. If you move 60’, you still go 50 mph. Not my rule…
You create the Portal to go in and at the end of the spell you touch everyone and create the Portal to go out. This is also the case with Dismissal. Dismissal is a Standard Action, and in-effect just ends the spell by creating a portal right where you are. If you want people to go through the portal with you, you have to touch them.
OK – per the Manual of the Planes, whenever you create a portal (in or out), it remains for 1d6 x 10 minutes. Denizens of either plane can see the portal, and enter on a Will Save DC 15 – if they wish.
So, that means that when you leave either plane, the portal is left open behind you for a bit. It also means if you are attacked on either plane by a superior force, it is possible for the enemy to follow you through the portal, but they have to save DC15. It also means that if you ‘Dismiss’ the spell on the Shadow Plane, and leave Maythor behind, he can use the portal with a Will Save DC15.
I’m sure Cormyr has wizardry rules around this, so better check before you start using it all the time.