(Super) administrator of the project could add members to the project.
Method One:
Go to project setting page, click the [Project Settings] button in the right corner to enter the project setting page
In the project setting page, you could add or remove members in the [Project Members] column. This method is applicable to the setting of adding or removing batch members to the same project.
Method Two:
Go to the team page, click the avatar of the member which you want to invite, enter the member settings page and set the projects he/she participates in. This method is suitable for setting permissions for multiple projects of a same member.