This command is used to invite people to join a room. The server will send a message to the user inviting them to join and allowing them to enter through +i. This is the only way to let someone into a room that is +Iif they are not at least a UOP in or have bot access to it.
/invite residue #support
In general, if you have an invite only room, this is how you will let people join. You generally only want to invite people you know, because random invites are usually considered rude.