You can control when invitations and notifications get sent on each project. In the kebab menu (three vertical dots) in the top right hand corner of the project page toggle the 'Invite contributors promptly' control on or off. This can be seen in the screen shot below.
This control is on by default. This means email invites and notifications are sent as you add contributors. Turning this off means emails are held, then sent automatically 21 days after each contributor is added. Switching back on sends all currently held emails at once - effectively acting as a 'send' button.
On projects with Draft status nothing is sent until a funder verifies the project. If a project isn't verified within 21 days of a contributor being added, their details are deleted and they'll need to be added again. Privacy policy means emails can be delayed but not withheld.