When people exercise to gain weight.

Most of the people who walk through the doors of the fitness facility
tend to be guided by a sole aim:to lose weight.But every so often i
come acros another group of people on the opposite end of the spectrum
who,being underweight and generally skinny,face a major challenge in
trying to gain weight and muscle mass.
In the fitness industry jargon the term 'hard gainer' is used to
describe skinny individuals who find it hard to gain weight ,most of
whom possess a genetic body type scientifically referred to as
Ectomorphic.
Ectomorphs have the body type that is most often seen on the pages of
fashion magazines or the line up of elite athletes at various
international marathonr.They are slim at the waist and narrow at the
hips and shoulders,long limbed ,lithe and carry very little fat and
muscle.
Although Ectomorphs tend to be considered extremely lucky by their
heftier counterparts ,one must remember that they have problems.Female
ectomorps are likely to be flat -chested and may complain of looking
boyish or wish the had more womanly curve,while male ectomorphs
struggle to gain muscle and often feel taken for granted.
How can skinny person gain weight safely and effectively?