All Al‑Anon groups welcome new members
Unless otherwise noted, attendance at Al‑Anon meetings is limited to members and prospective members – anyone who feels their life has been or is being deeply affected by someone else’s drinking. A meeting listed as “Open” or “Speaker Meeting” may be attended by anyone interested in Al‑Anon/Alateen.
 
All Alateen meetings are “closed”, limited to members and other young people who feel their lives have been or are being deeply affected by someone else’s drinking. The only adults who may attend a closed Alateen meeting are sponsors who have been certified as AMIAS (Al‑Anon Members Involved in Alateen Service). Parents may not attend unless they are AMIAS.

You never need to make advance arrangements to attend a meeting; just show up and you will be warmly welcomed.
Although many meetings are in houses of worship, this does not imply affiliation. It usually reflects the fact that the rent there is affordable.