I think that is rather rude... they don't "need" you in the guild, but they "need" you to make up enough for a retreat... I wouldn't WANT to join that guild

Or maybe the membership is limited by space. But the person that invited you knows there may be a space opening up and if you go to the retreat the members will say "oh you remember her, she came to our retreat and was so much fun. We really want her to join."

The guild I belong to has a limited membership of 60. However, we do have women who come that are not members, and they come to every meeting and function. Visitors do not pay dues, but they have to pay $3.00 for each meeting they come to. There is a waiting list to become a member.