The generation of entanglement in mesoscopic atomic ensembles is a challenging task for the fields of quantum information, quantum teleportation and precision measurements. A few subsets of such collective entangled states are analyzed in this work: spin squeezed states, Schrodinger cat states and Dicke states. Environmental decoherence is also considered, as it affects the quality of entanglement and a simple formalism for treating cooperative decay of coherence of multilevel atoms is developed and exemplified in a two qubit case.