Members: Persons responsible and co-workers
Members are people that are in common with task or information specified in card.
You can see members of a Card in the Board's view.
If there are more several card members, you will see a number indication of them. Click on it to see them all.
Members will be visible also in a Card. Simply enter it to see more in the Card Members section.
The person with the first, biggest icon is a person responsible for a card. Other users are co-workers. To change a person responsible, click on other user's icons and select Make responsible.
You can also use drag&drop to move a co-worker into person responsible's place. The moved person will become a person responsible.
You can manage Card members in this section. See more here in Card user management article.