The star formation process is efficient in the sense that it produces stars with masses from a few times the mass of Jupiter up to a few hundred solar masses.
In a stellar cluster there are many low-mass stars and fewer higher-mass stars. If a giant star (like the Sun) meets a small-mass star (like an M-dwarf or a brown dwarf) then they may end up as a binary system, moving along together and rotating around each other, provided that they have a close interaction, meaning that they come within a few tens/hundreds of astronomical units from each other.
Even for high-density stellar clusters, those that contain a large number of stars per unit volume, the probability of having such close interactions is relatively small. Collisions between stars in clusters are even more rare (for a globular cluster this is expected to happen once every 10,000 years). If this unlikely event happens, for example in the centre of a cluster, then the smaller star will in effect be swallowed by the larger star with some energy being released during the impact.
Answered by astrophysicist Dimitris Stamatellos at the University of Central Lancashire