All of these are great recommentions but to me (who has started numerous bands and is definately not an expert), the most important thing to starting/replacing members are 1) Must be able to play. You don't want to spend your time teaching others how to play (there are exceptions) when there is work to do. A person should know his sh__, and 2) Even a weekend band will see each other a number of times in a month (gigs, practice, events, etc), you need to be to able to get along. You want to build a commaderie where we're all in this together. Unless all do their part, the gig doesn't work and you don't get that warm feeling when the song sounds great. All in all, there's no secret ingredient. It's a try, see if it works and go from there😬.