1；谢谢你能让我加入你们，成为你们的一员。（或者）2：很高兴能成为你们的一员。谢谢。
热心网友
很高兴能成为你们的一员,(I am) very pleased/glad to be a member of you/ to join you.It is a pleasure to be your group member.正式的：It is really a great hounor to become a member of you/that you gave me the chance.
热心网友
I am very glad to be one of you.:)
热心网友
谢谢你能让我加入你们，成为你们的一员。I appreciate the opportunity to be a member of yours.
热心网友
I am honored to join you. Thank you.这样完全可以了；没有必要搞得很麻烦。因为，即使用复杂的结构写出来，也不是外国人常用的表达法，感觉很不地道。
热心网友
1.Thank you for allowing me to join you and become one member of your group! or2.I'm quite pleased/glad to join you, thanks.第二句比较口语话,在和别人谈话是一般都使用第二句,第一句有点罗嗦.
热心网友
I'm very glad to be a partner of you all.Thanks.