Allow project manager to invite existing person.

Warning will be shown.

Change-Id: Iec3ef3cf11daf96bbf8dce75e8c171ec13eaa77c
Add explanation to 404.

Change-Id: I65a2c030c14400ee69772ca8ca44be2979b32188
Fix templates.

Change-Id: Ib860a811099d0e63bfe8617b5296ec62485de30f