QUEUE Priority

I have the following scenario to accomplish in my office.

I have 2 MB on my WAN and I want to divide the connection into 2 parts. First part is for executives (group1) and part 2 (group2) is for part time employees.
I want to make the settings in such a way, if group2 is not using the internet, group1 can take the 2 MB.
If group2 is using it, group one can take upto 1.5 MB and group2 can maximum take 0.5 MB.
If group1 is not using it, group2 can use it maximum for 1 MB

That is, I want to put big priority for group1 at any time over group2.

How is this possible on Router OS.