ground floor, get in on the - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Participate in the beginning of a venture, thereby gaining some advantage. For example, Investors were eager to get in on the ground floor of the new development. [First half of 1800s]