Are your sims at least best friends with the person they're trying to invite to join?
Yep. In both cases that I mentioned, the sims are both at 100/100, with a pink and a red heart.
There is a known issue that there will apparently be interference if you assign two clocks to the same room. Is that the case here?
For my instance, there is one bed in the room and one sleepclock, with only one owner assigned to the clock. In the first case I mentioned, the owner was away from the lot at work. For the second case, the owner was the one relaxing on the bed alone, and invited a visitor to relax.
I deleted both clocks and installed just one with 2 sims assigned to it. There are still 3 beds in the room but I haven't had any problems so far.
I did not delete the sleepclock after updating the mod -- I bought the sleepclock and installed it with a previous version installed. I can try deleting the clock and buying a new one to see if that makes a difference.