Invite people you know. It's always good to have people there you can count on if you need them.
Yeah, if you don't sing and can't really hold down things, make sure you can roust up a "House Band." If you build up enough steam and can afford to pay a house band a bit at the jam, or on a weekly "guest" to the open mic or jam, that'll bring in people more consistently and generally get better players.
There was a local bar that had a rotating host for an open mic night where I used to live, and I'd fill in as host every now and then. There's two local jams that I regularly go to, and two that I sit in on a whole lot as a house bassist. I liked the open mic because there wasn't as much bickering. It was just as fun to watch the guys bomb terribly as it was to hear some awesome 17 year old kid who blew everyone away with their sheer talent.
In addition to some great advice given on this thread, I'd also suggest that you actively attract and market to the best players that you can. If you have a jam, get a good group together as the house band. Once you have a handful of really good players playing at any give time, the ones who aren't as good will have more of a "learning" attitude instead of a "look-what I can do and I'm pissed that I'm not on yet" attitude. While you do some primadonna type musicians, in general, the better the musicians you get, the less attitudes and bickering you'll get. In general, the better the players you can attract, the better the jams go.
Be ready to put up with some ATROCIOUS and loud noise at times, especially at the jams. This'll happen at the jams even when good players are jamming out.
At the same time, they can be tons of fun, and you'll ALWAYS learn more while hosting. People are always bringing in great tunes, and you can get some great players who'll show you a thing or two. If you can pay enough attention to what works and what doesn't, your musicianship skills will rise a whole lot.
The networking can be really, really good, especially if you run a jam or open mic. You can get yourself known in the local scene really fast (for better or worse, depending on your talent and attitude) as "that guy who runs the jam/open mic at that bar on Tuesdays." If you have skills, you'll get sub gigs in no time, and can find other new band members fast, too. One of the jams that I go to regularly gets some great players. That jam directly attributed to the formation of some really good local bands. After moving into my new-ish town just about a year ago, I've now got sub gigs all the time. I got into a couple of of original bands (that shortly thereafter fell apart) and one really good cover band (which I just joined) because of how I'd sit in as the house bassist for a couple of jams. It was tons harder to get things going in the last city that I lived in before now, since there were literally no jams at all.
Also, there's times where things just magically click, y'all have a great time, and the audience is happy and amazed. I'd say "go for it." If you do and fail miserably, at least you can say you gave it a shot!