Joining The Amish
As previously mentioned, the Amish are somewhat hostile of outsiders attempting to join. That being said, some communities, particularly the Mennonite ones are fairly welcoming to outsiders. They must however, meet the full requirements for membership. The conditions vary depending on community, with the liberal and more moderate ones having a more relaxed policy, and the conservative ones being more rigid.
Leaving
Leaving the Amish community is a tricky thing. Not the process of leaving itself, as anyone can leave at any moment with total freedom, but the repercussions that occur. If the departure is short-lived, the community may even re-welcome you. However, a permanent departure will cause the leaver to be shunned by the community. The person will always be seen as an outsider and will never be allowed to participate fully in the community again. This is tough for some Amish folk as they will also be shunned by their families.