Data is accessed in a sharing group just as it would be if sharing did not exist. Trying to write to a READONLY view will cause an abend.
You can create a sharing group that permits programs in 24-bit addressing mode to access data above 16 megabytes. To do this, you would define the source in storage above 16 megabytes, and obtain a target in storage below 16 megabytes. Then initialize the source with data, so programs in 24-bit mode can share the data through the target.