How can I enable members to use the /function command

So you probably need to do custom triggers, like dropping a specific item, jumping five times, etc.

You could run a function when a player drops a specific item, with a repeating and always active command block. The command would be

execute @e[type=item,name=<name of item in quotes>] ~~~ function <name of function>

• Replace <name of item in quotes> with the name of the item you want to detect the drop of in quotes.
• Replace <name of function> with the name of the function that you want to run.