This paper considers a Gaussian multiple-access channel with random user activity where the total number of users $ell_n$ and the average number of active users $k_n$ may grow with the blocklength $n$. For this channel, it studies the maximum number of bits that can be transmitted reliably per unit-energy as a function of $ell_n$ and $k_n$. When all users are active with probability one, i.e., $ell_n = k_n$, it is demonstrated that if $k_n$ is of an order strictly below $n/log n$, then each user can achieve the single-user capacity per unit-energy $(log e)/N_0$ (where $N_0/ 2$ is the noise power) by using an orthogonal-access scheme. In contrast, if $k_n$ is of an order strictly above $n/log n$, then the capacity per unit-energy is zero. Consequently, there is a sharp transition between orders of growth where interference-free communication is feasible and orders of growth where reliable communication at a positive rate per unit-energy is infeasible. It is further demonstrated that orthogonal-access schemes in combination with orthogonal codebooks, which achieve the capacity per unit-energy when the number of users is bounded, can be strictly suboptimal. When the user activity is random, i.e., when $ell_n$ and $k_n$ are different, it is demonstrated that if $k_nlog ell_n$ is sublinear in $n$, then each user can achieve the single-user capacity per unit-energy $(log e)/N_0$. Conversely, if $k_nlog ell_n$ is superlinear in $n$, then the capacity per unit-energy is zero. Consequently, there is again a sharp transition between orders of growth where interference-free communication is feasible and orders of growth where reliable communication at a positive rate is infeasible that depends on the asymptotic behaviours of both $ell_n$ and $k_n$. It is further demonstrated that orthogonal-access schemes, which are optimal when $ell_n = k_n$, can be strictly suboptimal.
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