We were there last year during a peak time for tour groups from Brazil...seems like the peak is the first half of July. Overall, I found them to be pretty well behaved and I don't mind them singing as a group if it isn't disruptive.
However, although not their fault, they did play havoc with Fastpass returns (FP+ wasn't in effect yet) and waiting times, since they did have some very large groups. An example I use is that a large group (seemed like about 100) went to the FP return for Buzz Lightyear at MK, which caused the FP return line to extend outside the building. I would think the nature of FP distribution having an hour return time would be that the actual return would be somewhat distributed throughout that hour, but if a group has them that gets thrown off a bit by everyone showing up at once.
Same with the delay of posted standby wait times. Of course, we've all seen this happen even without tour groups, but a large group going into a standby queue will quickly throw off the posted time. The example I remember was a posted 10 minute time at Star Tours was considerably longer as the group was queued to just outside the building entrance.
It will be interesting to see if any of that changes with FP+. For example, will a large group be able to tie, for example, 100 tickets to a single
MDE and does the large group have the same rules of use 3 then get another. Perhaps Disney is trying to get groups into smaller chunks, thus more chaperones.
The green shirted CM's with the Brazilian flag were there last summer as well, and were there in early June this year, although I didn't see many tour groups from anywhere.