Inside Prices (Insert menu), you will find the Extra costs insertion section.

Just a few details that aren't mentionned in the documentation:

• only the FIRST INSERTED extra cost set as an extra bed (i.e. “ Consider this extra cost as an extra bed(s)? YES”) will be selected automatically when needed, thereby increasing maximum capacity of a room
• extra beds will be added automatically when necessary, but only if the biggest available room has a capacity smaller than what is needed (e.g. for a reservation of 3 people, Hoteldruid will first assign a room of maximum capacity 4, before assigning a double room with one extra bed)
• if you have rooms with different extra bed capacities, you need to create several extra costs with the following attributes:
  • for each extra cost, allow different maximum number of beds (i.e. for max 2 beds: “Multiply the cost by: Ask. Maximum number:2”)
  • make each extra cost incompatible with the rooms/rates you DO NOT want to be selected
• in case you have several extra costs set as extra beds (i.e. like the exemple above), be aware that automatic assignment of the extra bed will only work for the first extra bed cost that you have inserted

More info: please have a look at the Documentation section.