The only way I've been able to do it is have them comment on a post in your community, then click the three dots next to their comment. There's an option to make them an Admin.
There's a difference between admin and mod. If you appoint them as mod, they'll have permissions to moderate the community. If you're an instance admin and you'll click on "appoint admin", they'll be able to control settings of your instance.
While that's true, I just don't want any admins accidentally click on the wrong button when they try to appoint a mod on their own instance. Just correcting the wording, here.