That sounds like too many wires under the same screw. You might need an Intersystem Bonding Terminal Block/Bar (IBTB):
https://www.google.com/?gws_rd=ssl#q...nding+terminal
https://www.erico.com/catalog/literature/E803S-NAEN.pdf
http://www.erico.com/category.asp?category=R2431
https://www.erico.com/catalog/literature/E828B-NAEN.pdf
When you are adding other wires to the house electrical system ground, it is important not to disconnect the house electrical system from its own ground connection, even for a moment, for safety reasons. You should use a split bolt to add another wire to an existing wire that is grounded. It is also why the IBTB has a "lay in" feature for the house ground wire, so that it isn't necessary to disconnect it from its ground.
The coax shield should be grounded with a grounding block that is connected to the house electrical system ground with 10 gauge copper wire for electrical safety and to reject interference. For further compliance with the electrical code (NEC), the mast should also be grounded in a similar manner to drain any buildup of static charge, but the system will not survive a direct strike.
From my personal experience, using a grounding block to ground the coax shield is the most important thing to do. It will protect you from leakage current shocks from AC operated equipment.
CECB Leakage Current
Getting A/C voltage on converter box's antenna input !
http://www.digitalhome.ca/forum/81-o...ml#post1457594
Equipment Leakage Current
http://www.digitalhome.ca/forum/81-o...ml#post1457668
Do you know that there is a grounding thread on the Canadian forum?