Reporting Services DBs not appearing in Management Studio, but can be queried?
You can also use T-SQL

use master;
go

alter availability group [ag_name] add database ReportServer

backup database ReportServer to 
disk = N'\\some\shared\location\available\to\all\nodes\ReportServer_HADR.bak' 
    WITH FORMAT, INIT, COMPRESSION, STATS;
GO  

backup log ReportServer to 
disk = N'\\some\shared\location\available\to\all\nodes\ReportServer_HADR.bak';
GO


Now go to another replica and restore:

restore database ReportServer from 
disk = N'\\some\shared\location\available\to\all\nodes\ReportServer_HADR.bak' 
    with replace, norecovery;
GO

restore log ReportServer from 
disk=N'\\some\shared\location\available\to\all\nodes\ReportServer_HADR.bak' 
    with norecovery;
GO
alter database ReportServer set hadr availability group = [ag_name]
GO


However, what you really need to do is ensure that you're actually connected to the replica that currently holds the ReportServer* databases before you try adding things to the availability group. If you've somehow stumbled upon the secondary, or if you're connecting to the listener and the AG has failed over to what was formerly the secondary node (i.e. the replica that does not have the databases you need), then you're not going to see the ReportServer* databases. 

Finally, please make sure you read the documentation.