Don’t work in isolation.
You can be better if your friends make you better.
When you work alone, you are much less likely to learn new techniques and develop better habits. Often, developers that tackle projects by themselves lack knowledge of best practices. Also, they usually have lower quality standards because they only have to answer to themselves when doing refactoring.
On the other hand, a group of developers from various backgrounds can draw upon a much deeper collective pool of knowledge. If these developers work together and do some rigorous code reviews, there will be much more sharing of good ideas. Best practices from across the industry are more likely to spread when people work together and compare notes.
Solo developers tend to work on simple freelancing projects building basic websites that don’t integrate with other systems very much. Team developers are more likely to collaborate on larger, enterprise-level applications. To work on these larger systems requires a much broader skill set, and developers learn many of these skills by working in a friendly, collaborative environment.