JustGive uses the same nonprofit database that is maintained by Guidestar. If the organization is already listed in Guidestar (which is pretty much any 501(c)(3) organization), the charity is likely already in the JustGive database as well. To check, just visit http://www.justgive.org and try searching on the organization’s Tax-ID (EIN) number.
If the organization cannot be found on Justgive.org or does not come up in search results, tell the charity to visit Guidestar.org and register their organization on the Guidestar Exchange (it’s free). This will ensure that the charity shows up in search results on the JustGive website.