yeah with an 8 persn party each link gives a 60% bonus sp per kill instead of running it twice and getting 100% of that persons sp.

max links, the one starting +3, you get 100 for the starter +60% + 60% + 60% + 60%(last 60% is if you use guardians aspect). it keeps you from running it 4 times at 100% + 100% + 100% + 100% + 60% for that aspect on all 4. you lose overall sp by linking, but it keeps you from running all 8 leves for an 8 person party individually.

after you finish the first link on the leve the other 4 of the party links and you can run the same leve again. this lets you run the leve 2 times instead of 8 times saving alot of time. if you link your leve to the one that was started you do get you rewards for that leve, but you will only get gil for completing if you do not link. this allows a group to run 16 leves instead of having to run 64 individuals. so yeah, instead of getting 100% sp on each kill you gt 60% of the sp added onto the base when you link to anothers leve instead of the full sp from taking the time to run it twice.