If you glance at Odoo 14, you have only one mode "Edit Mode" used to share projects. There is a turn in the newly released Odoo 15 version, an additional access mode has been added to accompany Edit Mode. Readonly is a new mode designed to ensure safe project sharing.
If you follow the Readonly mode for sharing, you will be provided a link to access the Readonly mode to share the project. Besides, you can select Recipients to share projects. The key point is that no one can edit any project information in Readonly Mode.
Coming to an Edit Mode. In Readonly mode, you are given a link for hassle-free project sharing and restricted to edit any information mentioned in the project. On the other hand, you can freely edit information in Edit Mode. All you need to do is to select a few Recipients as invite people and can send them mail to invite for the project.
In Edit mode, only invited people have the right to access or edit information in the project.