Hey Renney,
Ugh, I totally feel your frustration with this one. It’s one of those things that should be simple, and it’s annoying when it doesn't work as expected. Thanks for linking that other thread—it immediately clued me in on the exact problem you're hitting.
You've actually put your finger on a known gap that we're all too familiar with. Right now, you can bring a guest in to collaborate, but the "Assign Task" button just doesn't want to play nice with them. It's a top-item on our team's fix-it list.
While the engineers are working hard on a permanent solution, here’s how most teams are working around it:
Assign the task to one of your internal teammates for now, and have them act as the point person to coordinate with the guest. It's an extra step, but it keeps things moving.
You can also try @mentioning the guest right in the Loop component. It doesn't formally assign it, but it pings them and makes it clear what they're responsible for.
I know it's not the perfect answer, but I hope it unblocks you for the time being. The best way to know when the full fix is out is to click the "Follow" button on that original forum post you found.
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If this was helpful for getting you back on track, would you mind marking it as the "Accepted Answer"? It really helps the next person who runs into this same wall.