The only thing I can think of is to play percussively. No drummer right? So who is going to lay a foundation for others to riff off of. In a sleepy tune like the one you posted above there is not much going on. A sparse and percussive rhythm guitar might add some punch. I could see it sounding mushy and muddy with a bunch of not so great players strumming a fairly loos and unstructured piece. You need to add the structure. Pretend there is a snare drum and chop a chord on that beat.
In you specific situation I believe less is more.
In you specific situation I believe less is more.