Being in a coven is to be with a group of likeminded individuals who help one another, it's fun and interactive, much kinder and more interesting than the main forums.
You can join a coven whenever you like, there are usually members of the coven that will help guide and have been on thier path for longer, while there are others who are new and need guidence, it is simply whenever you feel ready to join one.
It helps to be able to bounce ideas off people who are also following the same path, as well as share knowledge easily.
But of course some people prefer being solitary, I spent many many years practicing as a solitary practitioner and enjoyed it emensley, it is very freeing, but being in a coven allows you to come up with new ideas together, but you don't have to practice anything as a group if you don't want.
In a coven you can be 100% involved or only as involved as you want to be, you have the flexability so long as you comply with regulation.