#4 - Sept. 6, 2017, 1:55 p.m.
In theory you may enter the Greater Invasion Point more than once a week.
Yet you will still get a loot this way only once a week.
So while you may help your friends once you are done, it won't give you more rewards from the boss, same week.
Some more info about it
here.
You can also check the
WoWhead or
Icy Veins guides about it.