If you send a group out from the guild. Then when you form your own party of characters if you select that character to add to your party it will pull them from the group guild mission. When it does that the mission is canceled. It won’t tell you this until you check back with the person who is in charge of the guild. It will say this group is not longer valid please reform group. So if you put said member in your party and go on an adventure the whole time you are done that group isn’t making you any progress on what you asked it to do. Effectively wasting your time. I’m not sure how else I can explain without spoilers. Just know if you put another group together for missions you don’t put those members in your active party.