Managing admins & moderators of a Rust server might seem complicated, but it's pretty simple. The most important is to understand you can get the Steam64 of a player!
1 - Get the Steam64 of a player
Le Steam64 ID allows the server to recognize a player configured as admin or moderator.
The simplest way to get it is thanks to the Steam Community website. Indeed, each player as a profile page which address is formatted like :
http://steamcommunity.com/profiles/X
The X part in the address corresponds to the Steam64 of the player, which is precisely the ID you need to get to put it as admin on your Rust server.
2 - Add admin / moderator temporary
The fact to add an admin or moderator from the console panel or through RCON is generally applied for the current server session; any restart will only apply the configuration (look next part below).
You can add temporary admin with the command :
ownerid Steam64 "nickname" "reason"
moderatorid Steam64 "nickname" "reason"
3 - Add permanent admin / moderator
A Rust server doesn't generally have a full configuration system, except for admins/moderators.
Create this file on FTP if needed :
rust/server/mtxserv/cfg/users.cfg
Inside, you need to report the command for adding admins/moderators like in the previous section.
For admins :ownerid Steam64 "nickname" "reason"
moderatorid Steam64 "nickname" "reason"
Note: the "reason" is just a comment section to identify the reason why this Steam64 should be admin (ex: VIP access until a given date).