A team can have one of two access levels, open or private. Read more about how to change the team access level in Users and access.
Open teams are available to any user in the organization. Private teams are only available to those with specific access to the team.
To grant a user access to a private team, navigate to the team and select "Access" in the left colum menu.
From there you can add either a new user to the team, or an existing user aleady registered with the organization.
If you would like to add a user which you know already has a Wecomplish account within a different organization, use the "Add new user" option, but only fill out the email address field. The remaining fields will automatically be fetched from the existing user account when added.