While it is true that group boards are not as effective as they were in 2015, they can still help you reach a wider audience when you are starting from zero.
We don’t recommend that you spend all day promoting in group boards or communities as that is a huge waste of time. It is better to focus more on improving your own content and pins and making sure you create optimized boards and pin regularly to grow.
Consider using group boards as the cherry on top (not as the foundation). However, using it as an addition, yes, they are still worthwhile joining if they are in your niche. We don’t recommend joining group boards just for the heck of it. Keep in mind group board owners and communities can often take months to respond so until you wait for the acceptance, it’s a good idea to uplevel your content/pins in the meanwhile.