Company Owners or Company Admins can manage company members through Company Settings > Members.
There you are able to perform the following:
Invite new members;
Edit and Remove members;
Manage a list of member’s projects.
To add a new member use Invite Member option. Fill in the invitation form:
Role (choose between 4 roles: Company Owner, Company Admin, Regular Employee and Guest).
Boards (you can select Boards to which you are inviting a new member)
Also, you can Add Many Members at Once by separating emails using the Enter button. Note that in case you add a few people at once they all will be given Regular Employee Role.
As you click Invite, the user will get an invitation email with the activation link.
Use the Manage projects option to adjust the list of projects assigned to a user. You can remove and add new projects there.
You can also edit members' usernames or remove them by clicking the three dots menu in front of the user's name(by moving the mouse over it).