The draft is short and to the point and easy to understand.  The security considerations (and privacy considerations!) sections are well written and cover everything.  I'm just missing one clause.

The first paragraph reads:
   All of the contents of this Information Element are sent in the
   clear.  The containing Enhanced Beacon is not encrypted.

What I'm missing is "...and this is fine because the 6tisch-Join-Info structure contains no sensitive information."

I'm not disputing this or asking for rigorous proof, but it you say "this is sent in the clear", you should finish with at least a statement that says that this is OK.