What is the maximum rank difference for a competitive match?

What is exact rank cap for competitive mode in CS:GO?

For example: I have two friends, a Master Guardian I and a Master Guardian II. What will be lowest rank that will be able to play with them without getting full team(when lobby contains 5 players there seems to be no lock)?

When I was a silver 1 I wasn't able to play with my friend who was a silver elite master. So he switched his account to his silver elite account and we could play. So you can play with a person that has 5 ranks higher than you, but not 6 or above.

If you are in lobby with 5 players, there is no lock - you can be Silver 1 and your friend can be Global Elite, however if you are not teamed with 5 players, you are able to play ONLY if you are with your friend similar rank - the lock is pretty much few ranks. I dont have correct numbers, but for example, if you are Gold Nova 3, and your friend is Legendary Eagle, you are not able to play with him. But if you are Master Guardian 2 and your friend is still Legendary Eagle, you can play with him. I don't know correct numbers, but this is my experience.