It's a good practice to check if your guild mates can take on new challenges you bump into. If a monster is too tough or hitting too hard, it's not bad practice to return when you get stronger. The absence of levels for monsters allows one to take risks, and measure if they can take the challenge or not.
Even if guild members suffers fatal defeats, they wont be removed from your guild (even more since I didn't implemented hardcore mode yet).
Trying out new guild members you unlock is a good way of discovering their strengths and uses for battle, and looking for new ones helps too.
You never know when you'll bump into a new potential guild member.