No, it's a bug.

A backup of your npc file (the ones in your group) was taken since it failed to load. I will get a fix out during the day, which should allow you to restore this file and continue as normally.

Thanks Snejk you are a great modder. NPC mod is revolutionary and really fun. Thank you for making mod.

Just updated the mod to v0.3.7-2 which includes a fix for this.

To restore the backup:
Go to the save directory for the save you want to restore.
There will be a file called "player0_npcs.bin". To be safe, make a copy of this file as a backup.

Aside from this, there's another file called player0_npcs_<bunch of numbers>. This is the backed up version of your npc group. Take a copy of this one as well.

Remove the player0_npcs.bin file and rename the player0_npcs_<bunch of numbers>.bin file to just player0_npcs.bin, so the backup replaces the original file.

Now you should be able to load into your save again with your npcs still there.

Thank you Snejk! With your advice, I was able to successfully bring my friend Elias Taylor! Thank you so much Snejk!