Did they enlist in Canada or Britain?
Canadian records are fully digitized here:
https://recherche-collection-search.bac-lac.gc.ca/eng/help/pffwwLess is better in the search form, start with just first and last name, and with/without reg. number and ignore the rest.
If they enlisted in Britain, Ancestry.com has some records, but trying to view most of them will send you to Fold3, another pay site. FindMyPast has some as well.
Feel free to post names and numbers here, and we can take a peek of where they are, assuming they still exist. Many of the British WW1 records were destroyed in a fire.