Why is this always such an issue on MN?
It's not rocket science.
Do you know and eat out with the people regularly?
Yes - Unless the difference is vast, say over a tenner, Split the bill, it will even out over time.
No - Say at the start of the meal, 'are we splitting or paying separately?'
followed by 'I'm happy to do either' or 'I'd prefer to pay separately'
Then, everyone knows where you and they stand. You might find some want to split, (eg sharing starters, bottles of wine etc.) some want to pay separately, that's fine, the ones who want to pay separately do so, (but it's up to them to ask for a separate bill) and the rest split.
If you know and eat out with the people regularly but are a bit skint, you say at the start of the meal 'just to let you know, I'm a bit hard up at the mo, so I'm only going to order xxxx and I'll pay for my own'. Your friends will say 'ok, that's great' and work the rest of the bill out for themselves.
Really it's not hard. I've done all of the above at different times in my life, depending on friendships and finances. The thing is to let everyone know at the start, if people have invited you out, they want your company, and they really don't want to leave you feeling hard done by.
The only time it makes it more difficult is when those who want to have their own bills, start wittering, that they want to share a bottle of wine, But then you usually find that the ones who are more willing to split are also more likely to say 'oh for goodness' sake, you can have a glass of wine on me!' (just wanted to give a bit of love for the splitters as they are often seen as greedy, soulless, pariahs on MN, but we're really not, we just like things simple!)