It is pretty easy to add someone as a captain, co-captain, coach or even team manager. Here are the steps:
log into your TennisPoint account
Go to the "matches & teams" page in the top navigation
click on "add/edit & invite players" link
a. The add/edit players has a list of all your current players on the right side of the page
b. Find the player you want to make co-captain and click "edit" on the far right of the person’s name in the list
c. After you click the edit link, you will notice their information in the form on the top left part of the page
d. change the "Role on Team" to be co-captain and then click the “edit player” button and the form will clear and the player will show up in the player list on the left again with their new “role” (co-captain, captain, coach, etc)
FYI, a "team manager" is normally used for junior teams where the team mom/dad will manage the team but their name will not show up on the availability page nor the create lineup page.