The guild as a whole would likely mobilize. Erza would start formulating a strategic plan. She'd consider the strengths and weaknesses of Acnologia and figure out the best way to approach the situation. Gray would also be ready to fight, using his ice - make magic to support the rescue mission. They would all be united in their determination to get Lucy back safely.
Well, often in fanfiction about Lucy's brother, he could be a bit of a mystery figure. Maybe he has a secret past or some hidden powers. He could be someone who has connections to an unknown part of the Fairy Tail world. For example, he might be involved with a secret guild that has a big influence on the main story but has been kept under wraps. He could also be shown as having a strained relationship with Lucy at first, due to some past misunderstandings, but then they gradually reconcile as the story progresses.
Naruto would probably be very enthusiastic and friendly towards Lucy right away. He might be curious about her celestial spirit keys and ask a lot of questions. Lucy, on the other hand, might be a bit more cautious at first but would be drawn to Naruto's positive energy.
One common storyline is the reunion plot. Lucy's brother has been away for a long time, and when he returns, there are all kinds of emotions and revelations. Another is the protection storyline, where he devotes himself to protecting Lucy from a powerful enemy.
The guild would likely be ecstatic. Gray would probably make some snarky comment about Natsu finally growing up, but deep down he'd be happy for his friend. Erza would be all for it and might start planning the wedding immediately, giving orders to everyone in the guild to help out.
Lucy could be extremely excited. After all, finding Igneel is like stumbling upon a legend come to life. She would likely be in awe of his presence. She might immediately think about how this discovery could benefit her friends in the Fairy Tail guild. She could also be very respectful and try to approach Igneel in a friendly manner.
The Fairy Tail guild would probably be very curious about Naruto at first. They might be amazed by his strange abilities like running up walls and creating clones. Some members, like Lucy, would likely be a bit cautious, but also intrigued. Natsu would probably be eager to test his strength against Naruto right away.
Natsu would probably be really excited at first. He'd be all over the place, asking a million questions and making sure Lucy is okay. Happy would be right there with him, trying to help in his own little Exceed way. Erza would be very protective. She'd make sure Lucy follows all the proper precautions and would offer to train her gently so she doesn't lose her fighting edge during pregnancy. Gray might be a bit awkward at first but would soon warm up and offer his support, like helping with any heavy lifting around Lucy's place.
Happy would be sad and confused. He doesn't like to see his friends in trouble. He would fly around trying to gather information from different guild members. He would also stay by Lucy's side, trying to cheer her up. And all of her other friends in Fairy Tail would at least have some doubts about the decision. They know Lucy's character, and her sudden expulsion would make them question whether the guild made the right choice.