There are two basic systems in most guilds. Swap threads and 1.9 threads.
Swap threads are where you post your GB on a thread with a specific denomination, 20 FPs for example. You add 20 FPs to the last GB on the thread, then post yours which becomes the last one on the thread the next player adds to before posting their GB. This thread is good for early players with lower level GBs. It allows you to add to GBs in small amounts while having the possibility to earn blueprints on wanted GBs and reward spots, when earned, of course. Without earning rewards, this kind of thread is net zero, every FP you place on another's GB gets added back to your GB. When used right, it can be a net positive to your FP balance from the FP rewards you receive.
1.9 Threads are built around a Level 80 Arc GB, which multiplies the rewards received from contributing to other's GBs by 90%, meaning if a reward spot normally pays out 100 FPs, Arc 80 owners will receive 190 FPs for locking that spot. 1.9 threads are build around Arc 80 owners contributing 1.9% for each of the reward spots. If the Reward spot pays 100 FPs, the Arc 80 owner will pay 190 FPs on that spot knowing they'll get all those FPs back giving them a net zero loss/return on FPs. 1.9 threads are great for players with mid to high level GBs as it allows you to grab the maximum donations to their GBs, leaving them less to contribute. For those with Arc 80s, they get the rewards of extra blueprints to raise their own GBs, and extra medals to buy expansions to enlarge their city.
In a good guild, both types of threads can coexist with no issues. There are guilds however, that use them to exploit new players, having mastered the art of sucking FPs out of naïve, unsuspecting, guild mates.