See the permissions guide here to view what an Activity Leader can access and can do for you and your team.
To promote a volunteer to an Activity Leader:
Head to any Activity in your Activities Console that has a team
In the Activity summary screen scroll down until you hit the list of the "Team" Remember your team is the pool of volunteers that are associated mostly with that activity. For a reminder on how to build a team
Click Edit Team
Click on the volunteer you want to be an Activity Leader until it pops up with (Activity Leader) beside their name. You can have more than 1 Activity Leader per team, and you can also have an Activity Leader for a single volunteer activity. Also, your Activity Leaders can be Activity Leaders of more than 1 activity if need be.
If you need to remove this permission from someone, use the same steps as above until (Activity Leader) is not next to their name. If you click the red X on the left side of their name, this will remove them from the team.