For this sort of use (dangling over a session) I'd be tempted to splash out on a cheap large diaphragm condenser mic with twin diaphragms that has a pattern switch for 'cardiod' 'figure eight' and 'omni'. You don't need a studio quality mic for this; Behringer do a decent one called the C3, it costs £58 on ebay. Set to omni, it should pick up pretty much everything (so watch what you say between tunes!). I like the AS50, it's a cracking little amp for the money. You can also pick up a feedback destroyer on ebay for around £50, if you are having problems.
If you want to take this further and use a mixer, a couple of boundary mikes on tables will work well, as Bernard suggests. Used together with the condenser and mixed properly, you should get a reasonable coverage of the performers.
Tim