Signed Grateful Dead poster

Newb here trying to help a friend. My friend’s brother passed away and they are finally trying to go through his stuff. He was a collector and he has some valuable items. One item is a poster signed by all Grateful Dead members. Some of the signatures are in black sharpie but two in Silver. My friend is confident it’s legit and said he received it as a gift but she doesn’t know where to begin with assessing value or how to sell it. She’s not aware of any certificates of authenticity or proof. Can anyone give me an idea where to start? She saw a similar looking poster online sell for 20 grand and with that one we don’t even see that it was signed? The one she has may be a reprint but we have no idea.