...there are at least 10 people in the group. However, if there's a group leader, we'd have to...
In the first sentence, the word group is a noun. In the second sentence, it's used as an adjective: a "noun adjunct". Thus, the indefinite article a modifies the noun word leader.
You can remodel the sentence to make the word group a noun in the second sentence:
However, if there is a leader in the group, we'd have to..
Now, you do need the definite article before group.
Per Barmar's comment, you can omit the word group from the second sentence altogether:
...there are at least 10 people in the group. However, if there's a leader, we'd have to...
The reference to the group would be then implicit.