Articles confusion

[ChinYuan]

I know the general meaning and how to use them: definite and indefinite articles.

But I encountered some confusing articles in the following sentence below:

I am the new member of the volunteer group in your community. A group is quite large, and the two people have been members for three years.

Is there anything wrong about " A group"?

Thank in advance for your response.

[ckunnath]

I am a new member of the volunteer group in your community. The group is quite large, and two people have been members for three years.