You can use:
usermod -a -G grouptoadd username
Since it is far less obvious than I thought why -a -G is ok while -aG is not I'll try to explain it:
The -G options takes either one value or a list of values. In case of one value the value may be given like: -GVALUE since there is no requirement to have a space between the option and the value. This is just common practice.
If you have a group called a on your system, then calling -Ga becomes ambiguous. Do you want the user to have only one group a? Are you calling -a and -G? Then is "a" a value to -G? Where is the rest of the value (to -G)? It is also common practice that any single-letter option can before any other single letter option. so tat -xy == -yz. But this is ambiguous when one of the options can take either a single value like VALUE or many values like VAL1,Val2,val4,etc.
So in order to make the command FAILSAFE and work properly for all input you have to separate -a from -G
usermod - modify a user account
usermod [options] LOGIN
The usermod command modifies the system account files to reflect the changes that are specified on the command line.
The options which apply to the usermod command are:
Add the user to the supplementary group(s). Use only with the -G option.
-G, --groups GROUP1[,GROUP2,...[,GROUPN]]]
A list of supplementary groups which the user is also a member of. Each group is separated from the next by a comma, with no intervening whitespace. The groups are subject to the same restrictions as the group given with the -g option.
If the user is currently a member of a group which is not listed, the user will be removed from the group. This behaviour can be changed via the -a option, which appends the user to the current supplementary group list.