You can cast for others, it's just better if you do it yourself. If you hire someone online to cast, there's a great chance it's a scam because a. once they have your money, whose going to make them cast, b. they need personal information otherwise they cannot direct the spell [and giving out such information can be dangerous] and c. they aren't as desperate for it to work as you are. You want things to work out, they just want their paycheck.
Group castings are a good idea if you have a coven. Say someone needs a healing spell, the group can charge more energy than just one person. Usually, you should only pay when you are chipping in for materials. Once you understand about magick you can cast for yourself, and it's better in the long run if you take the time and learn rather than spending 100$ or more on a spell costing 5$ in materials.