Guest are able to:
View projects/tasks/time reports/invoices/estimates shared with them
View upcoming milestones
Create new Discussions and comment inside them
You can, however, give them extra rights from Guest settings → Privileges like the ability to:
Add tasks on their own
Guests CAN’T:
Be assigned to a task, nor assign users to a task
Modify task details for an existing task not added by them
Access the Files area of a project (see Note 1)
Access the Expenses module (see Note 2)
Track time
NOTE 1: Guests will still be able to access files under a task or task comment, as well as those within discussions.
NOTE 2: There’s one special case when guests can view expenses, and that is when an expense has been attached to an invoice that’s been shared with them.