Yes and No.
When we build Indy teams, we add enough players so that even with absences, you should be able to field a full team.
If your team does not have enough to field a full team (for example, 10 kickball or softball players), the Captain (once established) should submit an Indy Team Sub Request form: https://www.underdogseattle.com/page/IndyTeam-Substitute
If your team can still field a full squad without bringing a sub, a sub will not be allowed. Bringing extra people takes away from the playing time that others on the team have paid for.