5. Group the positive integers $1,2, \cdots, n, \cdots$ such that the $k$-th group contains $k+1$ numbers:
$$
\begin{array}{l}
\underbrace{(1,2)}_{\text {1st group }}, \underbrace{(3,4)}_{\text {2nd, } 2,5}, \underbrace{(6,7,8,9)}_{\text {3rd, group }}, \\
\underbrace{(10,11,12,13,14)}_{\text {4th group }}, \cdots
\end{array}
$$

Then 2019 is in the \_\_\_ group.
5.63.

It is easy to know that the last number of the $n$-th group is $\frac{n(n+1)}{2}+n$. When $n=62$, $\frac{62 \times 63}{2}+62=2015$. Therefore, 2015 is the last number of the 62nd group.
Thus, 2019 should be in the 63rd group.