Communicating with your wedding party for their loungewear is key, their opinion should matter. Ask what they might feel most comfortable in. Decide if this is going to be a gift you purchase for them or request they purchase.
And, choosing what you all will be comfortable in day of will be a bit easier to choose! Wedding day loungewear is not a set thing. Monogrammed shirts are not for everyone. How about the typical “BRIDESMAIDS” labeled robes. These sometimes are not ideal.
Things to consider:
Not everyone feels comfortable in the same thing. Chatting with your wedding party and presenting your wants gets the conversations flowing. You can have a group discussion or have each person letting you know privately what they would feel comfortable being in.