Kavod harav,
Someone was walking by a shul and they call him in to be the 10th man. When he comes inside, they are already holding by Yishtabach. By the time that he puts on tallis and tefillin, it is not possible to catch up, especially since the davening is fast, and he ends up missing tefilla b'tzibbur. The next time that he passes by that shul and gets called in, is he allowed to refuse to help them, or demand that they wait for him to catch up? Or should he help them and miss tefilla b'tzibbur?
Michael
In such a case he could refuse to help them since they are denying him the opportunity of being part of their Minyan. However, remember that even if he davens part of Shemoneh Esreh with them or even starts Chazaras ha'Shatz with them, it is still considered Tefilah b'Tzibur (according to some Poskim, even joining during Chazaras ha'Shatz is a form of Tefilah b'Tzibur), so he should consider asking them to wait with Barchu until he has put on Tefilin and then abridge his Pesukei d'Zimra by saying Baruch she'Amar-Ashrei-Yishtabach and join them.
If, however, he will not be able to have any Kavanah and there are others who could possibly complete the Minyan, he is also entitled to refuse to join them, as the most important thing is Kavanah.
Yoel Domb