"From twenty years old and upward, each one who goes forth into communal service in Yisrael." Bamidbar 1:3
"The term צבא in Scripture does not necessarily, or even primarily, denote an army, or service in an armed "host." In Numbers 4:3, כל בא לצבא לעשות מלאכה באהל מועד ["each one who comes to communal service to do (sacred) work in the Tent of Appointed Meeting"]; ibid., Verse 23 'לצבא צבא לעבוד וגו ["who comes to perform communal service, to minister..."], and also elsewhere in Scripture, it refers to the service performed by the Levites in the Tabernacle. Verses 24 and 14 in Chapter 8, too, prove that צבא denotes any group of individuals united for communal service under the orders of a higher authority, or the service to be performed by such individuals. In the present verse, too, צבא need not necessarily have the connotation of armed service. Rather, it would denote anyone under obligation to come forth from his private life and perform communal services whenever this is needed; hence, anyone on whom the community can rely upon to attend to its interests...."
R' Samson Raphael Hirsch, Commentary on Bamidbar 1:3, The Chumash, Judaica Press, Translation by Gertrude Hirschler