I usually use a tiny wired clip on condenser with a little battery pack as an ambience mic. It's an audio technica, maybe model AT803. Not sure
1) I can clip it to something on the wall and it's pretty much invisible
2) it's got a high pass filter
but I've done gigs without one and it hasn't been a problem. Maybe that's because four of the five members have a live vocal mic onstage.
One time I used the main mix as my IEM feed so I used an sm57 clamped to the main PA speaker stand and pointed it towards the guitar amp and drums. I knew they'd be low in the main mix. That worked nicely too.
Only the lead singer tries to talk to me during the show, to call out the next song if he wants to go off the planned setlist, but he never remembers he needs to be louder with me.
I've been using them less than a year so maybe it will sink in eventually.
You're lucky to be doing this in a group where everybody else uses them. I'm the only one in that group who uses them and I've met a lot of resistance. Gotta save my hearing somehow.
This is for a group where I play keys and rhythm guitar -- the 2nd band I just joined I'll be doing bass again

so I may have to change my set up for that gig.