Simple. Just rep everybody you can, starting towards the bottom of the list, until you run out for the day...usually you can only do 5 instances per day. Then as players come back, on a daily basis, they try to return the favor. But don't forget to set the number of rep point when the rep dialog box pops up and the system will only let you re-rep someone just every so often...sometimes it takes awhile before folks can rep you back.