How will I know when a participant joins my team?

There are 2 ways in which you can be updated on this:

If you are a Team Captain, an e-mail message will be automatically be sent to you when a team member joins your team.
You can also go to the Team Progress screen and review the Team Roster:

Select the Team Progress icon in the header of the Participant Center.
Or, if you are at the Participant Center, you can select the Check my team's progress link in the lower half of the screen.
You will go to the Team Progress screen. On the lower half of the screen, look at the Team Roster. Any new members will be visible in the list of names in the Name column.