I've attended a Mennonite church for awhile now and seriously considering joining. How to join is very simple - you take a class and agree to follow the beliefs and standards of the church. If the church members see evidence of the new birth and the Spirit working in you, they will allow you to be baptized and become a member.
However depending on the type of Mennonite church you're looking at joining, there are some hard barriers to overcome. It's very likely that you'll have to give up things that you don't believe are necessarily wrong, like buying clothing from the store, or working at certain jobs. Two things that I'll have to give up if I join my church are musical instruments and internet - I know why they believe like they do, but it will be hard to get used to! Also, more conservative Mennonites can be isolated and develop a culture of their own, so although they are very nice and welcoming of outsiders, it can be hard for someone who wasn't born Mennonite to ever feel like they quite fit in.