Let's say you create more than one character using "multiplayer." Is it possible to remove a character using "reform party," and then add them back into the party later on?

When I try this, the character just stands there forever with a blue circle underneath them, and nothing happens when I click on them.

I'm playing BG EE now, but I think I tried this in the original BG back in the day and had the same experience.

Is it possible to temporarily remove characters created this way? Or do you have to have them in your party for the whole time?

Unfortunately, without modding you won't be able to recruit them again after leaving them. Here is the mod to help you:
http://gibberlings3.net/tweaks/

What you should be looking for there is in the 'Content Tweaks' section - Improved Multi-Player Kick-out Dialogues

This will allow you to tell your custom party members to 'wait here' or go to a nearby meeting place.

If you don't want that, you can try using the CTRL-Q cheat, simply by hovering your mouse over the NPC and pressing the buttons.

The last option is to export them, do the quest where other NPC's are required, and import them back.

Hopefully this helps.

I’ll try that mod. Thanks.