You can sign up for the test any time you want to. There is no need to be a member of any group or anything like that.

Doing the preparation on your own may work, but don't expect it to be easy. My first Solaris certifications on Solaris 7 I took the full set of instructor led classes offered by Sun (System Admin I, System Admin II, and Network Admin) before taking the exams. To update my certifications for Solaris 8 and 9 I just did some self study (roughly an hour per day for a month) before taking the exams. However, that was after already being certified in a previous version and having several years of full-time job experience. I think if you don't have that kind of background passing the test without taking any classes would be very difficult. For certain you'd need to study several books and get a Solaris system where you could do lab exercises and so on to try to get experience to go with the book learning.