(Which means they can be assigned tasks...)
They can make changes to Whiteboards.
(Which means they can brainstorm with you...)
They can have conversations within Kerika, by chatting on individual cards, or the board itself.
(Which means they are always in the loop!)
Anyone who is a Team Member on any of your boards is considered to be part of your account team, which means they are a "trusted collaborator" for you.
Any Team Member can create a new board in your account, and if they did this, they would probably be the Board Admin for that new board.
But you are still the boss of your own account: as an Account Owner you will always have ultimate control over every aspect of every board that lives in your account.
A Team Member can try to invite someone to join a board, in the same way as a Board Admin would: by sending an invitation to an email address. However, a Team Member's invitation is automatically blocked until it is approved by the Board Admin.
Check out this video on people, roles and privacy.
This is done automatically by the system: Kerika generates the Request and sends it to the Board Admin, and when the Board Admin approves the request (which can be done using email or the Kerika application itself), the Team Member's invitation is released and sent out.