Moodle (AKA Course Site) allows you to restrict access to Activities, Resources, and course sections based on criteria like group membership only, or a combination of conditions, which may include group membership, date, or a grade on an existing item in the grade book.

See this information on how to create a group to use group restrictions.

Conditional Access for Activities and Resources

You are able to display different content to different groups of students, and even release that content at different times, all based on group membership. This conditional availability for individual Activities is defined from the Settings Screen for that item. This settings screen is displayed when you initially add an item and is subsequently available by clicking on the link to that item on your course page, then on Settings for that item on the top level menu:

For example, an Assignment has two sections on the Edit Settings page that control access:

Availability - The settings in this section define the time frame during which the students can submit the assignment. Note that students can submit after the Due date, but it will be marked as late. If, however, you specify and enable a Cut-off date, submissions will not be allowed after the due date unless an extension is granted by you. This does not hide the Assignment, it just controls when assignments can be submitted by students.
Restrict access - First, expand the Restrict Access section by clicking on the reveal arrow. Then, click on the Add restriction button, and chose a criteria to add a restriction based solely on Date, Grade, Group, User Profile item or Restriction set, in which you can build a set of logical conditions based on a combination of the other criteria.

An example to restrict access to a specific group.

In order to restrict access to a specific group, you must have already created at least one group in the course.

For additional information about that process, see this Moodle Doc.

Navigate to the Add restriction page as outlined above and choose, Group.

This will add access choices to the Restrict access section.

In this example, we simply want to limit accessibility to the students in the group named, Group A so choose Group A from the drop-down menu. Confirm that the other options are correct. Note that the eye icon in this context indicates if a student who does not meet the criteria for the item sees it greyed out, or it is completely invisible. Default is that the item is greyed out, if you wish it to be completely invisible, click the icon to hide the item to those students not in the Group A group. It will, however be visible to the students in that group. Scroll to the bottom of the page to save your settings.

Here is what it looks like on the course page for you:

Here is what it looks like on the course page for a student in the group:

Here is what it looks like on the course page for a student NOT in the group:

Conditional Access for a Course Section

To conditionally restrict access for a course section, turn on editing, click on Edit button next to the topic area name and choose Edit topic, and then open up the Restrict access section and click on Add restriction... button.

Nested Restrictions for Assignments with Different Due Dates for Different Groups

You can create one assignment for multiple groups with different availability, this is recommended to prevent more than duplicate assignments in your grade book for the same assignment.

You'll need to create restriction sets linked by an "OR" statement".

Here is a link to a video explanation of how to add a set of nested restrictions

Manual on/off Toggle of Visibility

You can still manually restrict access by "hiding" an item, but for the most part, this hides it for all students or participants. For participants with an editing role, like Teacher or T.A., Moodle uses two conventions to indicate and control the availability of an item.

For course sections, look for an eye icon. By default, an item is visible, but when you click the eye, a diagonal line is placed over the eye and the item is greyed out. These are both visible indicators to you that your students can not see the item. Clicking the eye icon of a hidden item or course section will again make it visible. Note that a hidden course section hides all of the items in that section, even if the individual items are set to be visible.
For those items linked directly on the course page and that can be manually hidden, choose hide from the edit menu on the course page.

Additional Information

For additional information about restricting access for Activities, including Assignments, Resources, and course sections, see the Moodle 4.1 documentation.