What is the proper way to label dialogue spoken by a group of characters?

When multiple characters are speaking the same dialogue simultaneously (as opposed to two characters speaking different dialogue at the same time), you will usually handle it with slashes.

There are times where alternatives are also acceptable. If there is a singular label that clearly expresses who is speaking, for instance, it is perfectly okay to use that.

Generally, as with all formatting advice, if your method is clear and does not distract the reader, it is most likely fine.