Project participation will not automatically limit your ability to discover matches in the entire Family Tree DNA database. However, there are two ways that taking part in a group project (or not) may impact Y-Chromosome DNA (Y-DNA) matches.
First, if you have taken a Y-DNA test then each 11/12 (a genetic distance of 1 at 12 STR markers) match will only be shown if you and the match both belong to the same group project.*
Second, if you choose to join a project you will be able to choose between comparing just against your project or comparing against the entire database. You can change this at will from the My Account – Match and Email Settings portion of your myFTDNA account. If you choose to restrict matching to your group projects, your contact information will only be shown to matches who are in group projects to which you belong. Likewise, if one of your matches has chosen to restrict matching to a group project, then they will only be shown on the match lists for those in their group projects.