To share, or un-share a lead with another Agent:
1. Open up a lead record and click on the Profile tab.
2. Scroll down to system info, where you'll see the Share w/Agents field. Click the X by the agent you are sharing with to remove them.
You can also take a shortcut to sharing your lead, by mentioning the other agent via notes. "Tagging" them will share the lead with them:
A shared lead may not have all of the same functionality at the dashboard who the lead was shared to.