Just to be clear, the previous version of Family Historian didn't support the witness role and AS supported it within days of Family Historian 6 was released. Therefore you'll only be to do this if you have Family Historian 6 (which I can see you do from your FHUG profile).
If, for example, you want to add witnesses to an existing marriage:
- Right-Click on the marriage fact in the properties window
- Click on Witnesses..
- In the Witnesses dialogue, click on Add...
- If the witness is an individual in your file then choose 'Add Individuals', if you just want to record their name then choose 'Name Only'
- Then either select the individual or type in their name
- Choose the role (witness, best-man, etc.) and then click OK and add more if you wish
The same methods can be used for baptisms, burials and other facts.
Best wishes
Nick