I think all of our ADRs (including Beaches & Cream, IIRC) have given us the option to add one to our party of 7 (we haven’t needed it). It’s probably worth trying in a situation like @Jeff_AZ’s where you really are adding someone at the last minute, but I don’t think I’d risk counting on that when booking my original ADRs. Disney IT is notoriously inconsistent.
There are some places that I don’t think offer reservations for parties >6, so you’ll need to split the reservation into 2 parties and ask at the stand of you can be seated together.
Here is some data I pulled trying to get an idea of max party size a few months ago: