I once went for Salaat at a Masjid with a group and found that there is still lot of time left for the Jamaat. Since we were in a rush, I lead the prayer with the group with me. Is this not right?

In principle, for a second Jamaat to be performed there are different situations. The rulings for each situation will be different. Hereunder are the different situations:
a) The Musjid is structured solely for travelers
b) The Musjid does not have an appointed Imaam and Muazzin
c) The first Jamaat was performed by non-residents of the community d) The first Jamaat was performed by residents of the community. However, the Jamaat was performed without calling out the Adhaan.

The ruling for the above four situations is that it will be permissible to perform the second Jamaat with Adhaan and Iqaamat.
a) The resident of the locality called out the Adhaan for the first Jamaat. b) The first Jamaat was performed exactly like the normal Jamaat, i.e. from the front, etc.

In the above 2 situations to perform the second Jamaat is Makrooh Tahrimi.

If we analyse the above situations, the scenario which was explained falls under the category 'C' where non-residents of the community performed the first Jamaat. Therefore, it is better if the second Jamaat be performed from the official Musallah.

Lastly, we advise that if Salaat has to be performed before the stipulated time of Jamaat, it be performed in the Sahn (outer section) of the Masjid. Otherwise, perform in one corner of the Musjid, not at the place of the Imaam.

and Allah Ta'ala Knows Best

Mufti Ebrahim Desai