Hi Shameer

One way of achieving this is to create groups of users in the course and then assign the group to a Grouping

Each activity in a course can then have a Grouping selected for it under Common Module settings > Groups under advanced you can select the Grouping and select available for group members only.

To enable the groupings feature select Site Administration > Miscellaneous > Experimental > enable Groupings

This allows access to be set for each activity.

Another way possible in V1.1 of Totara on a course wide basis is to  Create cohorts with the members in each cohort. These could be created as dynamic cohorts based on a value on a user profile field.

The cohort could then be assigned to a program which has dates set for the courses.

Does Group A need to work on the  course during the second week if not then the courses could be set up as separate weekly recurring courses and cohorts assigned to the course

For more info on progarm management see http://help.totaralms.com/Creating_Editing_a_Program.htm

For info on Cohorts see http://help.totaralms.com/Cohorts.htm

Hope this gives you a few ideas on how this can be implemented.