Overview
You become a Quaker by joining a meeting. Quakers encourage newcomers to spend some time getting familiar with the Quaker way and with the community before making up their minds to formally join. You may spend anywhere from a few months to a few years as an “attender,” participating in worship and other meeting activities before you feel ready to make a commitment. (Some choose to be active attenders for a lifetime.) The first step toward membership is to write a letter to the clerk of the meeting expressing your wish to join formally. The clerk or a member of the appropriate meeting committee will be pleased to explain the membership process to you, but they may wait for you to take the first step, since Quakers are often reluctant to make someone feel pressured to join.
Source: Frequently Asked Questions about Quakers, Friends General Conference