why do users show up as numbers in multi-select field

Those numbers are the raw / true Quick Base User ID. Is there a reason that you're using a standard text multi-select versus a user-list field? Are you populating the values in this list from another field? A user-list will serve you better in the long term if you could convert it to that. 

Yes im populating this from another field,  Im using the multi-select because I need to be able to select multiple.

I guess is a missing feature or arguably a bug.  Just create a new field on that table with a formula of

UserToName([Userid])

Then use that as the source for your shared field.