Going to answer out of order, to make my response flow a little better.
When using it to transport willing participants, you would be one of the 8; you could either send 8 others or yourself and 7 others along with you. This is because it says it targets 'up to eight...' not 'you and eight...' and no spell targets the caster for free (barring some special circumstances, which this does not indicate).
If one of 8 (or less) creatures in a circle decides they are no longer willing (I'm looking at you Superstitious Barbarian), that creature would roll their Save to negate the spell in respect to themselves, allowing the other (up to) 7 to continue without them. This is because one creature Saving (Failing, or willfully Failing) against an area spell does not Save/Fail for other creatures affected.
If you have more than 8 willing, how the spell responds would be up to the DM because of 'Rule 0' but you can expect them to cite Spell Failure because your 'target' is invalid. They may allow you to transport the nearest 8, or select up to 8 out of the circle to affect, though.
Finally, if you have more than 8 and some are willing but others aren't, it is still within the realm of the GM but with a couple different options. The most likely responses by the GM (in no particular order) are:
- Allow the spell to transport the (up to) 8 willing creatures, giving the target option to the spellcaster, because there are (up to) 8 and they are holding hands in a circle, regardless of who else is in the circle. This functions I indicate in paragraph 1.
- As above, but instead of selecting only willing creatures, grab the nearest (or random or however the GM decides) creatures (regardless of will) and have any unwilling creatures attempt to Save. They could treat this as in Paragraph 2, or use this last bullet...
- The Spell fails, as Paragraph 3 because the 'target' is not what the spellcaster indicated, similar to if you attempt to cast and Evocation spell at a creature that has recently... say... Plane Shifted and is no longer in 'range'.
On your last question, you'd have to ask your GM. RAW, it doesn't indicate that everyone in the circle has/gets to go, so you could leave someone behind intentionally, but it is implied that the entire group travels as one, and it could be argued that leaving someone behind would 'break the circle' causing the invalid target situation I've mentioned. If you're trying not to tip off another player, you could send an IM/text the GM or ask them for a moment to the side. Most GM's will accommodate you.