its a bit complicated. but for easy way you can try to join some free servers. There are bunch of Rexuiz servers in servers list usually marked with "Best" word. Pick your preferred game type and join. Survival/coop/ffa(akimbo/non-akimbo/mixed arts). some of servers are free some have players. Just make a decision you wish to play alone or with other players.
But game also have ability to create your own server. Use Multiplayer window -> Create, create server you like, name it some how unique and start server. Supply name of the server to your friend and if everything is good your friend could join to your server using servers list and clicking on your server