How do I pin a post in a group

Becky • I have 2 beautiful boys 1 being an angel and hope that one day 3 might be the number for me . Otherwise 2 beautiful bubs 1 I have a little longer to wait be be able to hold is a gift no matter what ✨✨. Xo
I have created a group and would like to know how I pin a post-it is a description of the group and what I hope to achieve with it. Thanks so much in advance.