I’ll try to précis this from a long story. The problem appears to be common, and various not-entirely-convincing solutions are all over the Web in various help forums. Everybody has different setups, the Microsoft techs don’t really seem to understand how Teams works, and it seems to change every week anyway. This is what worked for us, but your mileage may vary.
When connecting with guest clients outside of our own organisation for online training sessions, we were finding that the ‘meeting chat’ panel (which, confusingly, has the tool tip ‘show conversation’) was absent for some users, or was dimmed out and produced the popup message, “You need to be a member of the team to use this”. The obvious solution seemed to be to have the clients join a Team set up at our end, and hold each training session within a Channel. No… as well as inconveniencing the learners at the client’s end, who now had to go through all the approvals and verifications needed to join our Team, now the chat panel didn’t work for anybody.
The solution actually came from the client’s IT people, but I thought I’d share it. The meeting should be set up in the Teams calendar as normal, but NOT as part of a channel. Bingo. We have the chat panel back, learners can use it as a backline to ask questions, and we can run polls. Happy bunnies all around.