(Super) administrator of the project could add members to the project.
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.
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.