Guild missions will contribute to the guild, as well as help you level up, along with putting some silver in your bank. Everyone in the guild is eligible to participate (unless your contract has expired, or you are on the newly implemented "leave of absence" status). If you open up the guild window, on the 3rd tab lists the missions we can choose from. There are combat missions, Life skill missions, sea monster hunting missions, and boss subjugation missions. Those that participate in the mission, get rewarded with silver (the more you contribute, the higher the pay, which can be collected by clicking "collect rewards", also located at the bottom of that tab). at level 52, I'd focus primarily on either life skill missions, or an easier enemy combat mission (like wandering rogues, then move up to helms or abandoned iron mine workers if you can).
If there is a mission you would like to do, just ask in guild chat if there's an officer on to accept it for you. However, please keep in mind that when doing some of the "large" or "extra-large" missions, it would be a real pain to try and complete on your own, so it is advisable to post in the guild chat and get a feel if anyone is interested in helping with the mission. Also, there is a cost to the guild for accepting missions, so we try to only accept missions we intend on completing. This way, members and the guild bank balance both benefit 😉. Hope this helps, and I look forward to joining in some missions with you! ~Compii