I joined a super active union (everyone is above level 30, most of them 40-50s and has played in the past 2 days, no worthless leechers with 20+ days unlogged here) and I make note of people on my friends list who join a lot and join them in return. Also, I have an alarm set to the burst times of my union, so I try and play then when I can (I usually catch the one in the evening, on my days off I can do the morning one). Burst times make you AND your union mates more efficient. I'll often try and join union members on friends list during burst times cause I know we'll both start with burst and do huuuuuuuge damage at start.
I also make sure to do a lot of damage to the boss, which tends to attract people. Very few people are interested in doing 15 minutes of work because you started a boss and went afk after doing like 3 hits to him. Also, running andro/cass is always more attractive because then people know if they join your raids or you join theirs, they get heals, which is important. I'm always much more reluctant to join people with d'art/solomon/arthur/joan because I know I'm carrying a lot of extra weight there. Even if their teams are amazing they can't offer me much help. Mordred and Sieg are ok though cause they give a lot of benefit to you (Mordred has a lot of OP afflictions and Sieg tears through rage mode), just not as good as Andro.
Oh, and I use stamps a lot to encourage active people. Go Go! Nice Work! Thanks!