I'm also a fan of this plugin and we use it a lot on my server. My experience with it:
The moment a player places a portal, only he can see it. The other players present can only see it after a new login.
Finding this entry point is really not easy and the "assist" isn't really helpful either.
We helped ourselves by first placing a few blocks in the place that border a free area of a block (2 blocks high so that you don't step over them).
Then go into the gap (i.e. walk in front of the wall) and place the portal. You can also make this entry a bit nicer, because it should stay if you want to find the entry point like this.
You can only delete portals if you have found the entry point and click on "edit portal" and then "delete" at the bottom.
But it's the same as when you create one, it only disappears visually the next time you log in.