a letter of invitation 邀请函
an invitation card [ticket] 请柬
admission by invitation only 凭柬入场
at the invitation of 应…的邀请
an invitation to sing [to membership] 应邀唱歌[入会]
accept [decline] an invitation (to a party) 接受[谢绝]邀请(参加聚会).

She received several invitations to Xmas parties. 她接到好几个圣诞派对的请帖.

A door left unlocked is an open invitation to burglars. 门不上锁等于开门揖盗.