"Hello,
1) You must activate the signature from Admin Panel => Users & Groups => Users >> click on Users options => Allow Signatures: All members then hit Save.
2) In your profile >> click on Preferences look there Always attach my signature: make sure is in Yes then hit Save.
3) Just edit your first post then you see Attach signature (signatures can be changed in profile) make sure is in Yes then Send.
and tell your members to do the same.
hope it helps"
Let's try.