He probably told the girls that they have to practice at the crack of dawn , on the night before.

It isn't very polite to wake up someone at the Crack of Dawn ( especially girls) so I suppose the girls were forewarned and woke up early.

He could have an early-riser girl get them.

Perhaps the easiest way for Oliver Wood to wake the girls on the Gryffindor Quidditch team would be for him to find a Gryffindor girl who happened to be up early and have her wake the girls on the team. Though most would not be, there would likely be at least one Gryffindor girl who was awake that he could send to wake the girls on the team. When he wakes Harry, on the way Harry sees Colin Creevey, who also mentioned hearing someone say Harry’s name on the stairs. Even if the student who said Harry’s name was a Gryffindor Quidditch player awoken by Wood, Colin was already awake, because Wood didn’t need him.

“Yawning and shivering slightly, Harry climbed out of bed and tried to find his Quidditch robes.

‘Good man,’ said Wood. ‘Meet you on the pitch in fifteen minutes.’

When he’d found his scarlet team robes and pulled on his cloak for warmth, Harry scribbled a note to Ron explaining where he’d gone and went down the spiral staircase to the common room, his Nimbus Two Thousand on his shoulder. He had just reached the portrait hole when there was a clatter behind him and Colin Creevey came dashing down the spiral staircase, his camera swinging madly around his neck and something clutched in his hand.

‘I heard someone saying your name on the stairs, Harry!”
_{^{- Harry Potter and the Chamber of Secrets, Chapter 7 (Mudbloods and Murmurs)}}

Wood only would need to find one Gryffindor girl who was awake, and have her go wake the girls on the team and tell them to come to practice.

He also could yell or set off the alarm.

While Oliver Wood would not have been able to actually go into the girls’ dormitories to wake the girls on the Gryffindor Quidditch team, he could have yelled at them from the base of the stairs to get up. Boys are allowed to approach the stairs to the girls’ dormitories - Harry and Ron get to the sixth stair before the staircase turns into a slide and sent them down. Additionally, when the staircase stops boys from going up, it is signaled with a loud wailing noise. Oliver Wood could have intentionally made this alarm go off to wake the girls.

“I wonder if Hermione’s seen this yet?’ Harry said, looking round at the door to the girls’ dormitories.

‘Let’s go and tell her,’ said Ron. He bounded forwards, pulled open the door and set off up the spiral staircase.

He was on the sixth stair when there was a loud, wailing, klaxon-like sound and the steps melted together to make a long, smooth stone slide like a helter-skelter. There was a brief moment when Ron tried to keep running, arms working madly like windmills, then he toppled over backwards and shot down the newly created slide, coming to rest on his back at Harry’s feet.”
_{^{- Harry Potter and the Order of the Phoenix, Chapter 17 (Educational Decree Number Twenty-Four)}}

He would not have needed to enter to successfully wake the girls for Quidditch practice. Either by yelling up to them or by setting off the alarm by going up the stairs, he could be loud enough that they would be woken by the noise. This would likely also wake up girls who were not on the Quidditch team, but with his singular focus on Quidditch, that likely would not stop him from doing it.

The girls did not seem to know before.

Though how exactly he got them up is unclear, it does seem clear that none of the other members of the Quidditch team, including the girls, were forewarned that they were required to be awake for Quidditch practice or were naturally awake that early. Except for Wood himself, none of the team seemed awake or prepared to have been awake so early in the morning for practice.

“The rest of the Gryffindor team were already in the changing room. Wood was the only person who looked truly awake. Fred and George Weasley were sitting, puffy-eyed and tousle-haired, next to fourth-year Alicia Spinnet, who seemed to be nodding off against the wall behind her. Her fellow Chasers, Katie Bell and Angelina Johnson, were yawning, side by side, opposite them.”
_{^{- Harry Potter and the Chamber of Secrets, Chapter 7 (Mudbloods and Murmurs)}}

Therefore, it seems reasonable to presume that the girls were not forewarned they would be having early morning practice, nor were any of them already awake.