Hi @Rostow, Jacquelyn (Spectrum EE),
Welcome to Q&A!
Based on official document (Get started with town hall in Microsoft Teams), a town hall can host up to 10,000 attendees or up to 20,000 attendees with Teams Premium.
It should function well if you would like to invite a distribution groups of 1500 members.
If I misunderstand your real question, please kindly share the screenshot of the messages popping up when you attempt to add the distribution group into the "Optional" line as well as some links you mentioned that might have conflict information in the Q&A site.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
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