If the host adds a project to an existing session and another user chooses to cancel the project sharing process, a dialog is displayed with the following text:
The session participants must be synchronized at all times. Declining an invitation will therefore eject you from the session. Are you sure you want to leave?
However, clicking Yes will actually keep you in the session and leave all previously shared projects as shared - which leads to the broken state described in bug #759. The dialog text does thus not match the reality of what really happens in any way.
With patch https://github.com/saros-project/saros/commit/d9bedcae2046492c763e0bf8e7e93ebe9089957f you are now kicked / leave the session.