I assume that these are the paragraphs to which you refer:
Meantime the markets kept ever growing, the demand ever rising. Even manufacturer no longer sufficed. Thereupon, steam and machinery revolutionised industrial production. The place of manufacture was taken by the giant, Modern Industry; the place of the industrial middle class by industrial millionaires, the leaders of the whole industrial armies, the modern bourgeois.
Modern Industry has converted the little workshop of the patriarchal master into the great factory of the industrial capitalist. Masses of labourers, crowded into the factory, are organised like soldiers. As privates of the industrial army they are placed under the command of a perfect hierarchy of officers and sergeants. Not only are they slaves of the bourgeois class, and of the bourgeois State; they are daily and hourly enslaved by the machine, by the overlooker, and, above all, by the individual bourgeois manufacturer himself. The more openly this despotism proclaims gain to be its end and aim, the more petty, the more hateful and the more embittering it is.
The use of "army" is a metaphor in these extracts. Marx is not advocating the formation of an industrial army, he is observing that the hierarchical nature of factory work is analogous to the ranks in an army, with the producers (ie the workers) being on the lowest rank. The army of industry is the workers in "average" jobs
Marx considered those who are in irregular, badly paid, temporary work together with unemployed to form a pool of people who could be "called up" in times of economic growth and dropped in recessions.
This is extended in other works to the observation that in bourgeois society there are often unemployed people (it was rare to be unemployed in feudal society) and the unemployed (and underemployed) fill the role of the "army reserves". Unemployment is a consequence of the bourgeois view of workers as a resource to be exploited, and would be eliminated in the Communist utopia.
This then ties back to the grand thesis: Work should be fulfilling, and it can't be unless workers control the means of production.