The Join This Group portlet instance is displayed on the public page of a public group's context. The way this portlet behaves varies depending on the enrollment option currently in place for the group.
For example:
If the group has open enrollment, with no application required, the portlet includes a Join Now button that lets you join immediately.
If the group requires a membership application, the portlet includes a link that says Click here to apply to join. Clicking this link lets you fill out and submit an application.
If the group is invitation only or if it is closed to new members, the portlet includes a message stating as much.
If you need help navigating to the public page of a group, see Browsing a list of public groups.
Note that it is possible for an administrator to rename the Join This Group portlet instance, but you can recognize its behavior -- it always behaves in one of the ways described above.