Spell it out....tell potential members and new collectors...what areas your group covers.
For example if you were moderating the Eclectic Electric Group...you might consider posting a list of what collectibles your group covers (toasters, radios, stoves, fans etc;). There are times when you need to spoon feed your group members.SPELL IT OUT for them.
By listing what your group covers you help the reader think about items that they may have or are interested in discussing.. Posting a list of collectibles covered in your group helps them understand more about the group. It is like providing inspiration for future posts.
It serves as a trigger to help them generate new content and photos.
So you might add a post on the comment wall and also a copy in a forum post in your group.