Disclosed are methods for an invitee to gain admittance to a group. An inviter already in the group and the invitee share a secret password. The inviter uses the password to create an invitation and then issues the invitation to the invitee and to an authenticator. The authenticator creates a challenge key and challenge value and sends the challenge value to the invitee. Using the password and information from the invitation, the invitee recreates the challenge key, uses the challenge key to derive a response value from the challenge value, and sends the response value to the authenticator. The authenticator compares the response value with an expected value and, if they match, knows that the invitee must have been able to recreate the challenge key. The authenticator trusts that this invitee must be the one for which the inviter issued the invitation and admits the invitee to the group.