Can I retain the party I build for the tutorial?

The tutorial will give you characters. In fact, if you create your own party, you will run into issues as you are forced to recruit them.

Creating an entire party is not the intended way to play but can be done. Originally this was via the multiplayer mechanics to create a single player multiplayer game but EE makes it much easier.

I would advise against it in your first playthrough however for a few reasons:

• There is no shortage of NPCs to recruit. You can have a full party extremely early in the game so having 6 from the start isn't helpful.
• The companion NPCs are all interesting. They have unique banter, interactions, quests, abilities, and in one case a hamster. The protagonist is also unique due to the plot but secondary player characters tend to be dull.
• Making a full, balanced party when you don't know what you're doing is difficult and time consuming.

Note that this does not apply the The Black Pits, which expects you to build an entire party from the start.

It looks like I misunderstood the user interface, and didn't see the option for continuing without creating a party.

1. It's not necessary to create party members for the tutorial
2. It's not necessary to create an optimised party for the tutorial
3. I don't know if there's any way to retain them, but you may as well just create the party you want in the main campaign.