I'm really glad you pointed this out - I am going from HI to Reds Meadow in September and was counting on Yarts for transportation from Mammoth to the Valley - I didn't see that they only operate on weekends after July - I just did a quick search and found a place called "High Sierra Shuttle" based out of Bishop - contact info is 760-872-3950 office/760.258.6060 cell
Here is what they say about themselves:
Getting around the Sierra's from trail head to trail head – quickly, affordably, and safely. We do it better than anyone. For over 8 years we have been a full service trail head shuttle service, serving the Eastern Sierra's from Cottonwood/ New Army Pass trail head to Happy Isle trail head located in the valley of Yosemite and every trail head destination in between. We have the right people to service your transportation needs at the right price. We are family owned; fully insured company based in Bishop, CA. that. offers a host of services that will take the hassel out of your transportation needs. Various publications such as Back Packer magazine and US Forestry advertize our company.
I have no idea what the cost is but if you decide to use them or when and if you find someone else, please post your experience so I can figure out what I will do too.
Here is another website with some alternatives - Unfortunately, it looks like it's going to cost us to get where we want to go
http://climber.org/data/shuttles.html