Instead, he was originally introduced as antagonist for the squad, which was formed under the leadership of Cable (making the juxtaposition in Deadpool 2 a bit funny).
That said, he not only eventually wound up joining the early version of the team, but has also been a part of multiple iterations since — one of the most notable being Rick Remender’s Uncanny X-Force.
In the comics, Deadpool has been closely associated with X-Force from the very beginning, his second appearance ever being in X-Force #2, but it should be noted that he didn’t actually start off as a member of the group.