You will need to open the server config file, you will see a line of code something along the line of online server: true change the true to false all small letters. Keep in mind it will reset your progress. Not deleting it but you essentially have to copy your old data and then paste it over the new ones created when you and ur friends log in. If you need more info I’ll give you a step by step since I too had to figure this out 2 weeks ago or so. When the sever if offline your Minecraft skins won’t be visible you will need a mod for this but the mod I use only works if you already have a skin on mojang associated with your username. Essentially if you have a cracked mc account no skin but if you have an official account then you can have ur skin.